test(api): add func-var seams and unit tests for registry_handler.go

[marwan562]

What

Closes #1066. pkg/api/registry_handler.go previously had zero test coverage — none of its functions could be unit tested without a real Harbor instance. This adopts the same func-var seam pattern already proven in member_handler.go (and extends it to the remaining 26+ handlers later, per the issue).

Changes

• Add listRegistriesFunc and viewRegistryFunc — package-level vars aliased to ListRegistries and ViewRegistry (mirroring member_handler.go:28-31).
• Route GetRegistryIdByName through listRegistriesFunc so tests can swap it.
• Route GetRegistryResponse through viewRegistryFunc instead of making its own direct client.Registry.GetRegistry call. This deduplicates the API call and lets GetRegistryResponse inherit ViewRegistry's not-found handling.

Bonus bug fix

The old GetRegistryResponse returned nil, nil (silent success with no payload) when the registry had ID 0 / didn't exist, because the err it returned on that path was the already-nil error from the successful call. Routing through viewRegistryFunc means a missing registry now returns the "registry is not found" error like ViewRegistry does.

Tests added (pkg/api/registry_handler_test.go, 5 tests)

• GetRegistryResponse when ViewRegistry returns an error
• GetRegistryResponse happy path
• GetRegistryIdByName when ListRegistries returns an error
• GetRegistryIdByName when registry name is not in the list
• GetRegistryIdByName happy path

No new imports or dependencies. No external API contract changes.

Verification

• go build ./... — ok
• go test ./pkg/api/... — ok (including the pre-existing member_handler tests)
• golangci-lint run ./pkg/api/... — 0 issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 9.71%. Comparing base (60ad0bd) to head (f7d16f9).
⚠️ Report is 223 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##             main   #1089      +/-   ##
=========================================
- Coverage   10.99%   9.71%   -1.28%     
=========================================
  Files         173     326     +153     
  Lines        8671   16524    +7853     
=========================================
+ Hits          953    1605     +652     
- Misses       7612   14781    +7169     
- Partials      106     138      +32     

Files with missing lines	Coverage Δ
pkg/api/registry_handler.go	13.08% <100.00%> (+13.08%)	⬆️

... and 239 files with indirect coverage changes

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