Refactor dead helpers and trivial guard wrappers; consolidate session ID utility

[Copilot]

Static analysis flagged dead production helpers in logger, no-op wrapper indirection in guard, and a single-function micro-file in server. This PR removes unused code paths and trims indirection without changing runtime behavior.

• Logger cleanup (dead production code)

  • Removed extractEssentialFields from internal/logger/rpc_helpers.go (unused in prod and tests after refactor).
  • Removed internal/logger/log_cleanup.go and moved its message-normalization helper into test-only scope (internal/logger/log_cleanup_test.go).
  • Updated internal/logger/rpc_helpers_test.go to match test-only helper naming and removed obsolete extractEssentialFields tests.

• Guard policy validation (wrapper inlining)

  • Removed trivial pass-through wrappers in internal/guard/wasm_validate.go:
    • validateStringArray(...)
    • isValidAllowOnlyRepos(...)
  • Replaced call sites in internal/guard/wasm_payload.go with direct config package calls.
  • Updated guard tests to call canonical config validators directly.

• Server utility consolidation

  • Moved truncateSessionID into internal/server/session.go.
  • Deleted redundant single-function file internal/server/session_util.go.

// before
if err := validateStringArray("trusted-bots", trustedBotsRaw, true); err != nil { ... }
if !isValidAllowOnlyRepos(reposRaw) { ... }

// after
if err := config.ValidateStringArrayField("trusted-bots", trustedBotsRaw, true); err != nil { ... }
if !config.IsValidAllowOnlyReposValue(reposRaw) { ... }