Add missing autocomplete test coverage

[FuJacob]

Summary

Adds broad unit coverage for Tabby's autocomplete support logic so request construction, prompt policy, normalization, reconciliation, focus capability scoring, overlay presentation, and async work cancellation are protected by tests. Also tightens ghost-text color hex parsing after the new value-model tests exposed that malformed strings could be accepted too loosely.

Validation

xcodebuild -project tabby.xcodeproj -scheme tabby -destination 'platform=macOS' build
# ** BUILD SUCCEEDED **

xcodebuild -project tabby.xcodeproj -scheme tabby -destination 'platform=macOS' build-for-testing
# ** TEST BUILD SUCCEEDED **

xcodebuild -project tabby.xcodeproj -scheme tabby -destination 'platform=macOS' -derivedDataPath .context/DerivedData test CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY=-
# ** TEST SUCCEEDED **; 149 tests, 0 failures

I also ran the normal signed xcodebuild test; it still fails before executing tests because the app-hosted test bundle and host app have different Team IDs locally: mapping process and mapped file (non-platform) have different Team IDs.

Linked issues

None.

Risk / rollout notes

Most of this PR is test-only. The only runtime behavior change is stricter six-digit RGB validation in SuggestionTextColorCodec.nsColor(fromHex:), which rejects malformed persisted or user-provided color strings instead of partially scanning them.