fix: prevent dry-run crash on clientAuthCredentials handler

[harshithRai]

🔧 Changes

Fixes a crash during import --dry_run that aborted the entire dry-run preview with handler.dryRunChanges is not a function.

The clientAuthCredentials feature added two handlers that were implemented for the deploy path only and did not define a dryRunChanges() method. Because the dry-run engine calls dryRunChanges() on every registered handler, the run crashed as soon as it reached this resource - regardless of whether the tenant actually used client credentials.

• Added a dryRunChanges() method to ClientAuthCredentialsHandler and ClientAuthCredentialsPreHandler, each returning empty changes ({ del: [], create: [], conflicts: [], update: [] }) - the same no-op shape the base handler returns when there is nothing to do.
• Client credentials are per-client sub-resources with no top-level asset list, so they are intentionally not itemized in the dry-run diff for now. Credential changes are still applied correctly during a real import.
• Hardened the dry-run loop to skip any handler that doesn't implement dryRunChanges (with a warning) instead of crashing, so a future deploy-only handler can't reintroduce this failure.
• A normal import (without --dry_run) was never affected.

No changes to the YAML/JSON config shape - the clientAuthCredentials / client_authentication_methods structure is unchanged. This only affects dry-run behavior.

🔭 Future scope

• Itemize client credential creates/deletes in the dry-run diff so the preview reflects the changes a real import would apply, rather than showing no-op.

🔬 Testing

Unit Test

• Added regression tests confirming both handlers return empty changes without crashing during dry run.
• Added a conformance test asserting every registered handler implements dryRunChanges, so a non-conforming handler is caught at build time.

Manual:

• a0deploy import --input_file=./tenant.yaml --dry_run now completes on a tenant with client credentials instead of crashing.

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.27%. Comparing base (7ebf407) to head (dc98a8f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1438      +/-   ##
==========================================
+ Coverage   80.25%   80.27%   +0.01%     
==========================================
  Files         163      163              
  Lines        7553     7559       +6     
  Branches     1667     1668       +1     
==========================================
+ Hits         6062     6068       +6     
  Misses        801      801              
  Partials      690      690              

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