refactor: semantic function clustering — consolidate rate-limit utils, httputil, and logger helpers

[Copilot]

Automated semantic analysis identified several functions in wrong files or packages. Implements all actionable items from the clustering report.

Changes

• httputil.ComputeRetryAfter (new export) — moved computeRetryAfter from proxy/handler.go to httputil/github_http.go, exported, and co-located with its companion ParseRateLimitResetHeader. Caller in proxy/handler.go updated; time import dropped.

• logger/global_helpers.go — absorbed initWithWarning and logFallbackWarnings from logger/init.go, which now only contains the public InitGatewayLoggers/InitProxyLoggers orchestration API. Removes init.go's import "log".

• httputil/endpoint_helpers.go deleted — WriteSimpleHealthResponse and WriteReflectResponse merged into httputil/httputil.go. File had only 2 functions wrapping WriteJSONResponse.

• Cross-reference comments — ParseRateLimitResetHeader and parseRateLimitResetFromText now point at each other; both parse rate-limit reset timing but from different sources (HTTP header vs. MCP tool result text).

Skipped: mcpresult → mcp consolidation

Would create a circular import: mcp → difc → mcpresult → mcp. Left as-is.

[Copilot]

Pull request overview

This PR refactors and re-clusters a handful of HTTP/rate-limit and logging helper functions so they live alongside their closest semantic companions (and removes a now-redundant helper file), improving discoverability and reuse across internal packages.

Changes:

• Promotes and moves the rate-limit ComputeRetryAfter helper from proxy into internal/httputil/github_http.go, updating proxy call sites and tests.
• Consolidates logger initialization warning helpers into internal/logger/global_helpers.go, leaving internal/logger/init.go as orchestration-only.
• Deletes internal/httputil/endpoint_helpers.go and merges its two small response helpers into internal/httputil/httputil.go, keeping behavior intact.

Show a summary per file

File	Description
internal/server/rate_limit.go	Adds cross-reference comments between header-based and text-based rate-limit reset parsing.
internal/proxy/rate_limit_test.go	Updates tests to use the new exported httputil.ComputeRetryAfter.
internal/proxy/handler.go	Switches proxy retry-after injection to call httputil.ComputeRetryAfter and drops the local helper.
internal/logger/init.go	Removes private helpers/imports so the file only orchestrates public logger init.
internal/logger/global_helpers.go	Centralizes initWithWarning / logFallbackWarnings helpers in a shared logger helper file.
internal/httputil/httputil.go	Merges health/reflect endpoint helpers into the main httputil file.
internal/httputil/github_http.go	Adds exported ComputeRetryAfter colocated with GitHub rate-limit header parsing.
internal/httputil/endpoint_helpers.go	Deleted; functionality moved into internal/httputil/httputil.go.

Copilot's findings

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• Files reviewed: 8/8 changed files
• Comments generated: 0