Refactor DIFC helper placement and extract response content-rewrite helpers from jqschema middleware

[Copilot]

This addresses two persistent structural outliers from semantic function clustering: a DIFC-only helper living in tool registration code, and a cohesive content-rewrite block embedded in the large jqschema.go middleware file. The refactor keeps behavior unchanged while tightening package boundaries and discoverability.

• DIFC helper colocation (internal/server)

  • Moved isSingularReadTool(toolName string) bool into difc_log.go with the rest of DIFC filtering/error helpers.
  • Keeps DIFC classification logic and logging context (logDifcLog) in one module; tool_registry.go remains focused on registration responsibilities.

• Content rewrite extraction (internal/middleware)

  • Extracted the rewrite cluster out of jqschema.go into content_rewrite.go:
    • rewriteFilteredTextPayload
    • rewriteEnvelopeTextPayload
    • rewriteFirstContentItem
    • rewriteContentItemText
  • Leaves jqschema.go centered on jq compile/apply flow and middleware orchestration.

• Tests and module shape

  • Existing middleware and DIFC tests continue to exercise these paths; rewrite-helper coverage remains in dedicated rewrite-focused tests.
  • Resulting structure mirrors prior payload.go/schema.go separation and improves future maintainability of filtering logic.

// internal/server/difc_log.go
func isSingularReadTool(toolName string) bool {
	singular := !strings.HasPrefix(toolName, "list_") && !strings.HasPrefix(toolName, "search_")
	logDifcLog.Printf("isSingularReadTool: toolName=%s, singular=%v", toolName, singular)
	return singular
}

[Copilot]

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