GitHub's commit API endpoint is rate limited to 60 requests/hour for
unauthenticated users. This causes avatar loading to fail after toggling
blame a few times.

This PR uses GitHub's CDN avatar endpoint
`https://avatars.githubusercontent.com/u/e?email={email}&s=128` instead,
which doesn't count against API rate limits. The author email is already
available from local git data (blame output), so
  no API calls are needed.

- When author email is available, constructs the CDN URL directly (zero
API calls)
  - Falls back to existing API-based behavior when email is unavailable
  - Adds unit tests for URL construction

  Closes #47590

  ## Test plan
  - [x] `./script/clippy` passes
- [x] `cargo test -p git_hosting_providers` passes (89 tests including 3
new ones i added)
- [ ] Manual test: Open a file, toggle git blame, verify avatars load
without hitting rate limits

  Release Notes:

- Fixed GitHub avatar rate limiting in git blame by using CDN endpoint
instead of API calls (#47590)