fix(cli): clear stale retry/loading state after cancellation (#21096)

[Aaxhirrr]

Summary

• Prevent retry attempt events from re-populating loading state after a canceled turn.
• Guard retry callback emission in core retry logic when the abort signal is already set.
• Show retry phrase and cancel/timer only while actively responding.
• Add regression tests for the cancel/retry race and idle loading UI behavior.

Testing

• npm run test --workspace @google/gemini-cli-core -- src/utils/retry.test.ts
• npm run test --workspace @google/gemini-cli -- src/ui/hooks/useGeminiStream.test.tsx -t Retry Handling
• npm run test --workspace @google/gemini-cli -- src/ui/hooks/useLoadingIndicator.test.tsx src/ui/components/LoadingIndicator.test.tsx

Fixes google-gemini#21096

[Aaxhirrr]

Closing this fork-only PR in favor of upstream PR: google-gemini#21960