Intern known request header names to static AsciiString - #2206

## Summary

`NettyRequestFactory` set three header values in forms that cost work on
every HTTP/1.1 request:

- A body-less POST, PUT or PATCH set `Content-Length` from an `Integer`
constant. Netty's header value converter calls `toString()`, so every
such request allocated a new `"0"` string.
- A body's length was boxed into a `Long` before the same conversion.
- The configured `User-Agent` is a `String`, so
`HttpHeadersEncoder.writeAscii` encoded it char by char through
`setCharSequence` instead of the `AsciiString` bulk copy.
`HeaderEncodeBenchmark` measured the same effect for header names
(#2206).

Changes:

- `Content-Length: 0` uses `HttpHeaderValues.ZERO`.
- A body length that fits in an `int` goes through `setInt`.
`HttpHeaders` has no `setLong`, so larger lengths keep the old path.
- The `User-Agent` is converted to an `AsciiString` once, in the
constructor.

## Behaviour notes

- The bytes on the wire are unchanged for all three headers.
`AsciiString` maps chars the same way the `String` encode path does:
Latin-1 chars become their own byte, and anything above U+00FF becomes
`?`. A test pins this.
- The validating request headers still reject an illegal `User-Agent`
(CR, LF and the like) with the same `IllegalArgumentException`, on every
request, as before.
- The `User-Agent` is now read from the config once per factory, the way
the cookie encoder already is. Only a custom `AsyncHttpClientConfig`
whose `getUserAgent()` changes between calls would notice.
`DefaultAsyncHttpClientConfig` is immutable.
- No public API change: Revapi passes.

## Benchmark

New `RequestHeaderValueBenchmark`: builds and encodes a request with the
factory's own headers, with each value in its old and new form. JMH
1.37, JDK 17.0.2, Windows, `-prof gc`, 3 forks, 5 x 1 s warmup, 10 x 1 s
measurement.

| Case | Before | After |
| --- | ---: | ---: |
| Body-less POST, `Content-Length: 0` | 817.5 B/op | 769.5 B/op |
| POST with a body, `Content-Length: 1536` | 841.5 B/op | 817.5 B/op |
| Writing the `User-Agent` value (`writeAscii` branch only) | 21.3 ± 6.2
ns/op | 11.1 ± 2.6 ns/op |

The host was 75-92% busy, so whole-request times (~800 ns/op) stayed
within noise. Allocation is the reliable figure. The `User-Agent` gain
was measured on the encoder's two branches in isolation. Neither branch
allocates.

## Tests

`NettyRequestFactoryHeaderValuesTest` encodes the factory's requests
with `HttpRequestEncoder` and checks:

- the default `User-Agent` is sent as configured;
- a non-ASCII `User-Agent` produces the same bytes as the `String` value
did;
- a `User-Agent` with CRLF is still rejected on every request;
- a caller-set `User-Agent` wins over the configured one;
- `Content-Length: 0` is sent for body-less POST/PUT/PATCH, and no
`Content-Length` for GET;
- body lengths, including one above `Integer.MAX_VALUE`.

## Verification

- `mvnw.cmd -B -ntp clean verify -Dgpg.skip=true` on JDK 17.0.2
(Windows): 1,919 tests, 0 failures, 0 errors, 35 skipped. Error Prone,
NullAway and Revapi passed. Local verify ran on JDK 17 because there is
no JDK 11 on this machine; CI covers JDK 11. `-Dgpg.skip` is set only
because gpg is not installed here.
- `RedirectConnectionUsageTest` timed out once under load. It fails the
same way on unchanged `main` and passed in the next runs.

Claude Code on behalf of Pavel Ptashyts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>