cap Content-Length before narrowing to int in Http2Client - #3431

`convertResponse` mapped only `-1` to `null`, so any other negative
`Content-Length` reached `Response.Body#length()` unchanged.

`HttpURLConnection#getContentLength()` returns the header verbatim once
it fits in an `int`, and already returns `-1` when the value is unknown
or above `Integer.MAX_VALUE`. Measured against a raw socket reply on
JDK 17, 21 and 26:

    Content-Length: -5           -> getContentLength() = -5
    Content-Length: -1           -> getContentLength() = -1
    Content-Length: 3000000000   -> getContentLength() = -1

Widening the guard to `< 0` therefore reports a malformed value as
unknown while leaving both the `-1` and the over-2GB cases untouched.

This is the same defect fixed for `Http2Client` in OpenFeign#3431 and for
`GoogleHttpClient` in the previous commit. `Client.Default` and
`Client.Proxied` extend `DefaultClient`, so they inherit the fix.

Covered by tests mirroring `Http2ClientContentLengthTest`.

Co-authored-by: kdelay <[email protected]>

`convertResponse` narrowed the response `Content-Length` to `Integer`
while only guarding the upper bound, so a negative header value was
passed straight through to `Response.Body#length()`.

That field is documented as "length in bytes, if known. Null if unknown
or greater than Integer.MAX_VALUE", and every other client already
rejects negative values before narrowing:

- `OkHttpClient` (`contentLength() >= 0 && <= Integer.MAX_VALUE`)
- `ApacheHttp5Client` and `ApacheHttpClient` (same guard on the entity)
- `Http2Client` (same guard, added in #3431)

`GoogleHttpClient` was the only remaining client without it, so a
malformed `Content-Length: -1` surfaced as `length() == -1` instead of
`null`. `InvocationContext` compares that value against
`MAX_RESPONSE_BUFFER_SIZE` to decide whether a response is bufferable,
and a negative length silently passes the check.

Adds the missing lower-bound guard and reads the header once instead of
twice. Covered by tests mirroring `Http2ClientContentLengthTest`.