feat(mcp): add username support for FalkorDB authentication

[david-morales]

Summary

Adds optional username parameter to FalkorDB configuration to support authentication in FalkorDB instances that require username+password authentication.

Problem

Previously, the MCP server's FalkorDB configuration only supported the password field, making it impossible to connect to FalkorDB instances that require both username and password for authentication.

Solution

This PR adds username support across the MCP server configuration chain:

• src/config/schema.py: Add optional username field to FalkorDBProviderConfig
• src/services/factories.py: Extract username from config or FALKORDB_USERNAME environment variable
• src/graphiti_mcp_server.py: Pass username parameter to FalkorDriver constructor

Configuration Example

database:
  provider: "falkordb"
  providers:
    falkordb:
      uri: "redis://localhost:6379"
      username: "myuser"          # NEW: optional username
      password: "mypassword"
      database: "my-graph"

Or via environment variable:

export FALKORDB_USERNAME="myuser"
export FALKORDB_PASSWORD="mypassword"

Backwards Compatibility

The username parameter is optional (str | None = None), maintaining full backwards compatibility with existing configurations that only use password authentication.

Testing

Tested with FalkorDB instance requiring username+password authentication. The MCP server successfully connects and performs all operations (search, add_memory, get_episodes).

[danielchalef]

All contributors have signed the CLA ✍️ ✅
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[david-morales]

I have read the CLA Document and I hereby sign the CLA