fix(auth-server): add histogram to disabled StatsD mock

[sreecharan-desu]

Because

• When config.statsd.enabled is false, key_server.js registers a no-op StatsD client in the DI container.
• Redis metrics code calls this.metrics?.histogram(...). Optional chaining skips null/undefined objects, but if metrics exists without a histogram method, the call throws at runtime.
• Reported in fxa-auth-server mock statsd lacks of function histogram  #16491.

This pull request

• Adds a createNoopStatsd() helper that implements increment, timing, histogram, and close.
• Uses the helper when StatsD is disabled in key_server.js.
• Adds unit tests covering the no-op client method surface.

Issue that this pull request solves

Closes: #16491

Checklist

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

How to review (Optional)

• Key files/areas to focus on: packages/fxa-auth-server/lib/noop-statsd.js, packages/fxa-auth-server/bin/key_server.js
• Suggested review order: read helper + test first, then the one-line call site change in key_server.js
• Risky or complex parts: none — behavior change is limited to the disabled-StatsD code path

Test plan

• yarn jest packages/fxa-auth-server/lib/noop-statsd.spec.ts passes
• Auth server starts with STATSD_ENABLE=false and redis histogram metrics do not throw