fix(retry): avoid replaying unread body data

[cprayer]

Fixes linkerd/linkerd2#15649

Background

When a retry starts with an empty replay buffer, replay_body remains set while the original body is polled. The first DATA chunk is forwarded and buffered, then replayed on the next poll within the same attempt.

Changes

• Clear replay_body when the replay buffer is empty and uncapped, before polling the original body
• Add a regression test verifying that an unpolled body is returned exactly once

Validation

• Regression test reproduces duplicated bytes before the fix
• All 24 retry crate tests pass on this branch
• Applying the fix and regression test to upstream a66af811 passes all 25 retry crate tests
• Docker reproducer: 10 runs, each with 29–30 duplicated bodies before the fix and zero afterward; consumed-body 503, early 503, and healthy-backend checks also pass

AI tools assisted with the investigation, reproducer, and PR description.

[anthoturc]

hey @cprayer, could you rebase your branch? it LGTM

[cprayer]

@anthoturc Thanks! Rebased onto the latest main

[anthoturc]

also for other reviewers, i was able to throw a slightly simpler version of the repro that @cprayer provided and confirm that the expected retry behavior is working properly.

kubectl -n replay-repro logs -f deploy/server -c server
2026/09/18 17:57:19 listening on :8080 (h2c)
2026/09/18 17:57:19 req 0: attempt 1, replying 503 without reading the body
2026/09/18 17:57:20 req 0: attempt 2, read 11 bytes "hello world" -- OK
2026/09/18 17:57:25 req 1: attempt 1, replying 503 without reading the body
2026/09/18 17:57:26 req 1: attempt 2, read 11 bytes "hello world" -- OK
2026/09/18 17:57:31 req 2: attempt 1, replying 503 without reading the body
2026/09/18 17:57:32 req 2: attempt 2, read 11 bytes "hello world" -- OK
2026/09/18 17:57:37 req 3: attempt 1, replying 503 without reading the body
2026/09/18 17:57:38 req 3: attempt 2, read 11 bytes "hello world" -- OK
2026/09/18 17:57:43 req 4: attempt 1, replying 503 without reading the body
2026/09/18 17:57:44 req 4: attempt 2, read 11 bytes "hello world" -- OK
2026/09/18 17:57:49 req 5: attempt 1, replying 503 without reading the body
2026/09/18 17:57:50 req 5: attempt 2, read 11 bytes "hello world" -- OK

With the fix applied, we can see that the server is only getting the original body as opposed to the duplicate (which would have lead to 22 bytes being read)

[raykroeker]

Thanks @cprayer and @anthoturc !