refactor: semantic function clustering — TLS relocation, util consolidation, and documentation

[Copilot]

Six housekeeping items from the semantic function clustering analysis: misplaced TLS helper, scattered TLS documentation, undocumented ResponseWriter embed pattern, single-function util file, missing layering comment, and an unexported-but-relocatable atomic write helper.

Priority 1 — Move configureTLSTrustEnvironment to httputil

configureTLSTrustEnvironment and tlsTrustEnvKeys were buried in cmd/proxy.go despite being reusable TLS infrastructure. Now exported from internal/httputil:

// Before: private to cmd/proxy.go
configureTLSTrustEnvironment(tlsCfg.CACertPath)

// After: in httputil, callable from anywhere
httputil.ConfigureTLSTrustEnvironment(tlsCfg.CACertPath)
httputil.TLSTrustEnvKeys // ["NODE_EXTRA_CA_CERTS", "SSL_CERT_FILE", ...]

Priority 1 — Cross-package TLS comments

Both httputil/tls.go and proxy/tls.go now carry package-level doc comments explaining the split: httputil owns protocol-level TLS helpers (version policy, config constructors, trust env); proxy owns cert generation (GenerateSelfSignedTLS).

Priority 2 — Document statusResponseWriter as pure embed

tracing.statusResponseWriter is a struct with no methods of its own — all behaviour comes from httputil.BaseResponseWriter. The doc comment now makes this explicit and cross-references the body-buffering server.responseWriter variant.

Priority 3 — Consolidate util/session.go into util/format_duration.go

session.go contained exactly one function (FormatSessionIDForLog). Merged into format_duration.go (renamed to "String and time/duration formatting helpers") and deleted session.go + session_test.go.

Priority 3 — Layering comment in config/config_env.go

Added a package-level comment explaining why config_env.go wraps envutil rather than calling os.Getenv directly: envutil provides generic typed accessors; this file adds gateway-specific validation policy on top.

Priority 3 — Flag atomicWriteFile for future relocation

Added a TODO comment noting that logger.atomicWriteFile should be exported to internal/util or a new internal/fileutil package once a second consumer outside logger appears.

[Copilot]

Pull request overview

Refactors shared TLS and formatting utilities while improving architectural documentation.

Changes:

• Relocates TLS trust environment configuration into httputil.
• Consolidates session formatting and its tests.
• Documents TLS ownership, response-writer embedding, environment layering, and future file utility extraction.

Show a summary per file

File	Description
internal/util/session.go	Removes consolidated helper file.
internal/util/session_test.go	Removes consolidated test file.
internal/util/format_duration.go	Adds session ID formatting helper.
internal/util/format_duration_test.go	Moves session formatting tests.
internal/tracing/http.go	Documents response-writer embedding.
internal/proxy/tls.go	Documents TLS package responsibilities.
internal/logger/fileutil.go	Notes potential atomic-write relocation.
internal/httputil/tls.go	Adds shared TLS trust configuration.
internal/httputil/tls_test.go	Tests TLS trust configuration.
internal/config/config_env.go	Documents environment-helper layering.
internal/cmd/proxy.go	Uses the relocated TLS helper.
internal/cmd/proxy_test.go	Updates tests for the relocated helper.

Review details

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• Files reviewed: 12/12 changed files
• Comments generated: 3
• Review effort level: Medium