Feature/modbus serial with generic transport (EPROT-74)

[MrS-E]

Description

Adds an opt-in generic RTU transport factory API for ESP-Modbus serial controllers.

• Adds mbc_slave_create_serial_with_transport() and mbc_master_create_serial_with_transport().
• Lets applications create and supply an initialized mb_trans_base_t through a documented factory callback for MB_RTU mode.
• Preserves existing mbc_*_create_serial() behavior by using the stock transport when no factory is supplied;

Motivation: I ran into a similar problem to issue #176 while integrating a custom serial transport, so I added a small, generic serial factory for custom RTU transports.

Related

Related issue: #176

This change provides an RTU transport-factory extension point for custom
communication and routing implementations, supporting the issue's goal of
reusing ESP-Modbus protocol/controller logic with application-provided I/O.

Testing

I tested it using my own transport implementation (I can't disclose here) that is wired to the esp-modbus component (via managed components) that is pinned to commit e74eeaa.
The wiring looks kinda like:

     #include "esp_modbus_slave.h"                                                                                                                                                                                  
     #include "transport_common.h"                                                                                                                                                                                  
     static mb_err_enum_t custom_transport_factory(                                                                                                                                                                 
         const mbc_slave_transport_factory_args_t *args,                                                                                                                                                            
         mb_trans_base_t **transport)                                                                                                                                                                               
     {                                                                                                                                                                                                              
         return custom_transport_create(args->comm_info,                                                                                                                                                            
                                        args->parent,                                                                                                                                                               
                                        args->user_ctx,                                                                                                                                                             
                                        transport);                                                                                                                                                                 
     }                                                                                                                                                                                                              
     void app_main(void)                                                                                                                                                                                            
     {                                                                                                                                                                                                              
         mb_communication_info_t comm_info = {                                                                                                                                                                      
             .mode = MB_MODE_RTU,                                                                                                                                                                                   
             .slave_addr = 1,                                                                                                                                                                                       
             .port = UART_NUM_0,                                                                                                                                                                                    
             .baudrate = 19200,                                                                                                                                                                                     
             .parity = MB_PARITY_NONE,                                                                                                                                                                              
         };                                                                                                                                                                                                         
         custom_transport_config_t transport_config = {0};                                                                                                                                                          
         void *slave = NULL;                                                                                                                                                                                        
         ESP_ERROR_CHECK(mbc_slave_create_serial_with_transport(                                                                                                                                                    
             &comm_info,                                                                                                                                                                                            
             custom_transport_factory,                                                                                                                                                                              
             &transport_config,                                                                                                                                                                                     
             &slave));                                                                                                                                                                                              
         ESP_ERROR_CHECK(mbc_slave_start(slave));                                                                                                                                                                   
     }

Additionally, I tested it shortly by using the slave example.

I did not test the master.

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.

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