[yusiwen]

What changed

agent/retry_http_test.go, one test:

• retries is now an int32 read with atomic, because the retry path and the test goroutine both
  touch it;
• the assertion is retries <= 1 instead of retries == 0, with the reason in the comment: if the
  cancel lands after the retry decision but before the backoff's sleep observes it, OnRetry announces
  the attempt the sleep then abandons — and no second request follows, which is the property the test
  is named for. requests == 1 keeps its stricter check.

Why

This is the flake filed as #295, and it just failed again — master's post-merge run for 8d1ccb8
(Test repeated in one process) failed with OnRetry fired 1 times, want 0. #295's acceptance says to
decide which of the two readings is true and make the test say it; the measured ordering says the
announcement is allowed, so the assertion moves rather than the code. The alternative reading — forbid the
announcement — would mean adding a cancellation check before announcing, which does not remove the race
(it would move it one line earlier).

Verification

• go test ./agent -run TestCancelledRunMakesNoFurtherRequest -count=50 → ok.
• go test ./agent -count=3 (the gate that failed) → ok.
• go test ./... -count=1 -race → ok.
• git log --format='%h %G? %s' → G.

Honest scope

The change makes the test tolerant of the ordering that the code actually exhibits; it does not make the
outcome deterministic. A run that does retry would still fail requests == 1, which is the property the
test exists for.

Refs #295