tests: add coverage for provider-specific fallback notices (#855)

[dknauss]

Follow-up tests for PR #858, which landed without test coverage.

What

Two tests for the conditional notice logic introduced in #858:

• test_user_two_factor_options_mentions_recovery_codes_when_backup_codes_are_available — when Two_Factor_Backup_Codes is registered, the profile UI should name "Recovery Codes" specifically
• test_user_two_factor_options_uses_generic_wording_when_backup_codes_are_filtered_out — when Two_Factor_Backup_Codes is removed via the two_factor_providers filter, the UI should use the generic fallback wording instead

Also adds two private helpers used by these tests (and available for future notice-related tests):

• reset_profile_errors() — clears the static $profile_errors array between tests via reflection
• render_user_two_factor_options() — captures user_two_factor_options() output to a string

Why

The fix in #858 is a conditional: one branch names "Recovery Codes" explicitly, the other uses generic wording. Without tests, either branch could silently regress. These tests pin both sides of the condition.

🤖 Assisted by Claude Code

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Co-authored-by: dknauss <dpknauss@git.wordpress.org>

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