Precompiled 2026-07-29-b7c47d7

Address review follow-ups from #55324 (#55411)

Follow-ups from review on #55324:

- Drop `std::env::set_var` from tests
- Prove 5xx and rejected-fetch retries share one attempt budget
- Link sources for network error codes

GitOrigin-RevId: be1fac06115b308fc3f0ae9c5928f58e83ca79ef