feat: add TOTP MFA support and improve Auth test coverage

[demolaf]

This PR adds TOTP multi-factor authentication support and improves Auth test coverage with better error handling.

What changed

• TOTP Multi-Factor Authentication: Added support for TOTP MFA with configuration and serialization
• Session Cookie Improvements: Refactored createSessionCookie to use SessionCookieOptions for better type safety
• Fixed error code consistency: use invalidUid instead of invalidProviderUid in getAccountInfoByFederatedUid for better semantic accuracy
• Improved JWT decode exception handling in verifySessionCookie
• Fixed session cookie JWT exceptions and integer division issues
• Added comprehensive unit and integration tests for Auth methods including verifyIdToken, verifySessionCookie, updateUser, updateProviderConfig, and getUserByProviderUid
• Improved test patterns with production-safe helpers and emulator safety checks
• Fixed Dart analysis type inference warnings by adding explicit type arguments
• Improved AuthProviderConfig sealed class usage to fix casting errors
• Refactored toGoogleCloudIdentitytoolkitV1MfaFactor methods for better readability

[github-actions]

Coverage Report

✅ Coverage 70.66% meets 40% threshold

Coverage: 70.66%
Lines Covered: 3429/4853

Minimum threshold: 40%

[codecov]

Codecov Report

❌ Patch coverage is 59.58702% with 137 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (next@8321902). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...irebase_admin/lib/src/auth/auth_config_tenant.dart	48.57%	72 Missing ⚠️
...ebase_admin/lib/src/auth/auth_request_handler.dart	44.04%	47 Missing ⚠️
.../dart_firebase_admin/lib/src/auth/auth_config.dart	86.48%	10 Missing ⚠️
...rt_firebase_admin/lib/src/auth/token_verifier.dart	0.00%	7 Missing ⚠️
...rt_firebase_admin/lib/src/auth/tenant_manager.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             next     #114   +/-   ##
=======================================
  Coverage        ?   70.20%           
=======================================
  Files           ?       67           
  Lines           ?     4829           
  Branches        ?        0           
=======================================
  Hits            ?     3390           
  Misses          ?     1439           
  Partials        ?        0           

Flag	Coverage Δ
unittests	70.20% <59.58%> (?)

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