fix(recovery-phone): use reg limit consistently across phone flows

[toufali]

Because

• The recovery phone registration limit was applied in some setup paths but not when changing an existing number, leaving the check inconsistent across flows.

This pull request

• Applies the existing getCountByPhoneNumber() limit check in changePhoneNumber(), matching setupPhoneNumber() and confirmSetupCode().
• Adds a unit test covering the registration-limit case.

Issue that this pull request solves

Closes: FXA-14069

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.

[Copilot]

Pull request overview

This PR makes the recovery phone registration-limit enforcement consistent by applying the existing “max registrations per phone number” check to the changePhoneNumber() flow and adds a unit test to cover the limit-reached case.

Changes:

• Add registration-limit enforcement to RecoveryPhoneService.changePhoneNumber().
• Add a unit test asserting changePhoneNumber() rejects numbers that exceed the registration limit.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
libs/accounts/recovery-phone/src/lib/recovery-phone.service.ts	Adds the max-registrations-per-number check to the change-number flow (throws RecoveryPhoneRegistrationLimitReached).
libs/accounts/recovery-phone/src/lib/recovery-phone.service.spec.ts	Adds a unit test for the registration-limit rejection in the change-number flow.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (1)

packages/fxa-auth-server/lib/routes/recovery-phone.ts:675

• In the unexpected-error path for changePhoneNumber, the backendServiceFailure call reports the operation as confirmCode, which makes logs/telemetry misleading for this route. It should identify the failing operation as changePhoneNumber.

        throw AppError.recoveryPhoneRegistrationLimitReached();
      } else {
        throw AppError.backendServiceFailure(
          'RecoveryPhoneService',
          'confirmCode',