test: cover stream known-size initialization race

[kernelpanic888]

PR #14541 (commit 44d40a3d8c) has already fixed the race by initializing .chunked inside Body.stream before the producer starts. This PR now keeps only the missing regression coverage for that implementation.

The tests verify two boundaries:

• a Body.stream producer observes .chunked immediately;
• a producer-provided .fixed 42 size survives completion of Response.Builder.stream and is not overwritten.

The branch is merged with current master; it no longer adds or changes the Body.stream API.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Mathlib CI can not be attempted yet, as the nightly-testing-2026-07-25 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Mathlib CI should run now. You can force Mathlib CI using the force-mathlib-ci label. (2026-07-25 18:29:29)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase d53dcb222f8122be00cfc00947e0154b9bb337c6 --onto 0bfc3acaef4ed0576307a77fbaa0c6e1a5dca402. You can force Mathlib CI using the force-mathlib-ci label. (2026-07-30 21:05:02)

[leanprover-bot]

Reference manual CI status:

• ✅ Reference manual branch lean-pr-testing-14552 has successfully built against this PR. (2026-07-25 18:34:35) View Log
• 🟡 Reference manual branch lean-pr-testing-14552 build against this PR didn't complete normally. (2026-07-25 18:35:53) View Log
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase d53dcb222f8122be00cfc00947e0154b9bb337c6 --onto a39eab69e1eee9ad38f4efe507907b1026a77808. You can force reference manual CI using the force-manual-ci label. (2026-07-30 21:05:04)

[kernelpanic888]

Resolved the conflict with current master by adopting the already-merged #14541 implementation. The PR is now intentionally test-only; the unique regression coverage remains, while the duplicate API change is gone. Keeping it in draft until the refreshed CI completes.

[kernelpanic888]

changelog-no