feat: added ProjectConfigManager, removed dynamicLinkDomain, missing error codes for Auth exceptions and tests

[demolaf]

Changes

• Add ProjectConfigManager for managing project-level auth configuration (email privacy, SMS regions, password policies, MFA, reCAPTCHA, mobile links)
• Comprehensive testing - unit and e2e tests for Project config features.
• Improved error handling - Added missing error codes and better exception messages
• Public JSON methods - Exposed toJson() for UserRecord and config classes
• Examples - Added projectConfigExample() and tenantExample() to demo app
• Implemented new linkDomain property with removal of dynamicLinkDomain
• Added comprehensive tests across all email action link methods
• Added unit tests for UserRecord, UserMetadata, UserInfo, MultiFactorInfo, MultiFactorSettings, and PhoneMultiFactorInfo covering edge cases, null handling, error conditions, and serialization

[github-actions]

Coverage Report

✅ Coverage 62.55% meets 40% threshold

Coverage: 62.55%
Lines Covered: 2895/4628

Minimum threshold: 40%

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 10 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (next@331f4d9). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...ackages/dart_firebase_admin/lib/src/auth/user.dart	9.09%	10 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             next     #111   +/-   ##
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  Coverage        ?   57.92%           
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  Files           ?       65           
  Lines           ?     4501           
  Branches        ?        0           
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  Hits            ?     2607           
  Misses          ?     1894           
  Partials        ?        0           

Flag	Coverage Δ
unittests	57.92% <16.66%> (?)

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