fix(settings): show rate-limit message on passkey 429

[dschom]

Because

• When a user clicks Create in Settings > Security > Passkeys and the auth-server rate-limits the request (errno 114 / HTTP 429 from POST /passkey/registration/start), the UI surfaced a misleading red banner reading "System not available. Try again later." — implying a server outage rather than a client-side throttle. QA filed this as must-fix before prod release.

This pull request

• Detects rate-limit errors in the PagePasskeyAdd catch block by checking errno === 114 or HTTP code === 429, mirroring the existing pattern in MfaGuard for OTP requests (packages/fxa-settings/src/components/Settings/MfaGuard/index.tsx:154-160).
• Routes the rate-limit case through getLocalizedErrorMessage() so the user sees the localized "You've tried too many times. Please try again {retryAfter}." (or generic) string from the existing auth-error-114 / auth-error-114-generic FTL keys.
• Skips Sentry.captureException for throttle responses since they are expected behaviour rather than exceptions.
• Emits a new rate_limited Glean reason on passkeyCreateSubmitFrontendError so the metric distinguishes throttling from real server errors. The existing reasons in this metric (not_allowed, timeout, etc.) are also free-form strings.
• Adds three jest cases covering 429 with retryAfterLocalized on begin, 429 generic on begin, and 429 on complete.

No backend changes. No new FTL strings. Same fix covers both /passkey/registration/start and /passkey/registration/finish since the catch block is shared.

Issue that this pull request solves

Closes: FXA-13666

Checklist

Put an x in the boxes that apply

• 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-settings/src/components/Settings/PagePasskeyAdd/index.tsx — the rate-limit branch in the catch block.
  • packages/fxa-settings/src/components/Settings/PagePasskeyAdd/index.test.tsx — three new test cases.
• Suggested review order: source change first, then tests.
• Risky or complex parts: none

How to Test

• Open rate-limit-rules.txt
• Set passkeyRegisterStart to passkeyRegisterStart : ip_uid : 1 : 10 minutes : 10 minutes : block, which makes testing eay.
• Login or singup.
• Once on the setting page, click add pass key. You can then cancle
• Repeat, you'll see the rate lmitting error message