Clarify logger fallback contract and add direct fallback-handler coverage

[Copilot]

The logger package already centralizes initialization through initLogger, but the fallback handlers for file, markdown, and tools loggers shared the same degraded-mode contract without documenting it explicitly. This change makes that contract explicit and adds direct coverage for the concrete handlers so fallback semantics do not drift.

• Fallback contract

  • Document loggerErrorHandlerFunc as the boundary for degraded-mode behavior:
    • fallback-capable handlers return a logger with useFallback=true and nil error
    • strict handlers return the initialization error
  • Clarify the per-logger fallback modes in initLogger docs, including ToolsLogger

• Focused handler coverage

  • Add direct tests for:
    • handleFileLoggerError
    • handleMarkdownLoggerError
    • handleToolsLoggerError
  • Assert each handler returns a usable degraded logger with the expected fallback state instead of relying only on filesystem-failure tests

• Example

  // Fallback-capable handlers return degraded mode, not hard failure.
  logger, err := handleMarkdownLoggerError(assert.AnError, "/tmp/logs", "test.md")
  require.NoError(t, err)
  assert.True(t, logger.useFallback)

[Copilot]

Pull request overview

This PR documents the “degraded mode” contract for logger initialization error handlers (fallback vs. strict behavior) and adds focused unit tests to ensure the concrete fallback handlers continue to return usable loggers with the expected fallback state.

Changes:

• Document loggerErrorHandlerFunc contract: fallback-capable handlers return (logger(useFallback=true), nil), strict handlers return an error.
• Clarify initLogger docs for per-logger fallback modes (including ToolsLogger).
• Add direct unit tests for handleFileLoggerError, handleMarkdownLoggerError, and handleToolsLoggerError.

Show a summary per file

File	Description
internal/logger/fallback_handlers_test.go	Adds direct tests asserting fallback handlers return degraded loggers with useFallback=true.
internal/logger/common.go	Documents the fallback/strict contract and clarifies initLogger fallback semantics per logger type.

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• Files reviewed: 2/2 changed files
• Comments generated: 3