extproc: always redact credential header mutations in debug log

[Aias00]

Description

The "request headers processed" debug log only ran header-mutation redaction when Server.enableRedaction was on. With debug logging on but redaction off (the aigw run default), the upstream filter's injected Authorization (the backend API key) was logged in cleartext, even though the incoming authorization header was already redacted by filterSensitiveHeadersForLogging.

This was inconsistent with the RequestBody phase, whose redactRequestBodyResponse always redacts header mutations and only gates body-content redaction on the flag. This change aligns the RequestHeaders phase to the same pattern: redact header mutations whenever debug logging is on, independent of enableRedaction, and gate only body content on the flag via a new redactBody parameter on redactProcessingResponseRequestHeaders.

A nil-response guard is added matching redactRequestBodyResponse.

Special notes for reviewers (if applicable)

• No behavior change to the response sent to Envoy; redaction only touches the logContent copy used for the debug log. Process-level tests assert on the returned resp, not log content, so they are unaffected.
• Verified live: with AIGW_DEBUG=true and enableRedaction off, the upstream Authorization mutation now logs as [REDACTED] while the actual response sent upstream still carries the real key.
• Out of scope: Envoy's own [http] and [ext_proc] debug logs still emit the raw Authorization header / proto and are outside extproc's control; those require Envoy log-level tuning, not addressed here.

Related Issues/PRs (if applicable)

Adjacent to but distinct from #2428 (which covers request body content redaction for the 7 passthrough endpoints). This PR covers a different leak surface: the backend credential injected into the request-headers header mutation by the extproc upstream filter.

Co-Authored-By: Claude noreply@anthropic.com

Partially addresses #2436 (extproc Go side; Envoy-side leak points remain).

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.92%. Comparing base (f9a03db) to head (1a8c172).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2435      +/-   ##
==========================================
+ Coverage   84.86%   84.92%   +0.06%     
==========================================
  Files         154      154              
  Lines       22419    22423       +4     
==========================================
+ Hits        19026    19043      +17     
+ Misses       2237     2224      -13     
  Partials     1156     1156              

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