fix(http): truncate streaming responses to content length (#36496)
Fixes #36456
## What
- Truncate streaming Deno.serve response chunks to the remaining
declared Content-Length.
- Finish the fixed-length response and cancel the excess stream once
that length is reached, including zero-length responses.
- Cover both the normal and shared HTTP/1 response writers.
- Verify that truncated fixed-length responses do not corrupt a reused
HTTP/1 connection.
## Why
The Deno-owned HTTP/1 path rejected an oversized stream chunk before
writing any of it. When the first stream chunk exceeded Content-Length,
clients received the response head but zero body bytes. This restores
the pre-2.8.3 behavior by writing the valid prefix up to the declared
length.
## Validation
- ./tools/format.js --check ext/http/http_next.rs
tests/unit/serve_test.ts
- deno lint tests/unit/serve_test.ts
- CARGO_INCREMENTAL=0 cargo clippy -j 2 -p deno_http -F
deno_core/quickjs --lib -- -D warnings
- CARGO_INCREMENTAL=0 cargo test -j 2 -p deno_http -F deno_core/quickjs
--lib (45 passed)
- CARGO_INCREMENTAL=0 cargo test -j 2 -p deno_http_h1 --lib (83 passed)
