feat(cron): add min_interval_seconds config to prevent excessive cron scheduling

[darrenzeng2025]

Summary

This PR implements the feature requested in #1655 to add a minimum interval configuration for cron scheduling to prevent excessive task scheduling.

Changes

Configuration

• Added MinIntervalSeconds field to CronToolsConfig struct
• Environment variable: PICOCLAW_TOOLS_CRON_MIN_INTERVAL_SECONDS
• Default value: 60 seconds (1 minute)
• Value 0 disables the check

Validation Logic

• Added validateMinInterval() function to check schedule intervals
• For every type: directly compares every_seconds with minimum
• For cron type: calculates the interval between next two ticks using gronx library
• One-time schedules (at type) are not affected

Error Messages

• Clear error messages indicating the minimum allowed interval and actual interval
• Example: "interval too short: minimum allowed is 60 seconds, got 30 seconds"

Tests

Added comprehensive test cases in pkg/tools/cron_test.go:

• TestCronTool_MinIntervalSeconds: validates default 60-second minimum
• TestCronTool_MinIntervalSecondsDisabled: validates that 0 disables the check
• TestCronTool_MinIntervalSecondsCron: validates cron expression interval checking
• TestCronTool_AtSecondsNotAffected: validates one-time schedules are not affected
• TestCronTool_MinIntervalSecondsCustom: validates custom minimum values
• TestCronTool_MinIntervalSecondsNegative: validates negative values use default

Related Issue

Closes #1655