Reject bare CR in managed HttpListener request parsing

[Dev-next-gen]

While reading the chunk-size change in #132747 I noticed that the managed HttpListener silently drops a CR that isn't followed by a LF, and glues together the text on either side of it. I replayed a few raw requests against the managed listener (Linux, .NET 8.0.30) and against http.sys (Windows 11, .NET 8.0.29):

request contains	managed HttpListener	http.sys
Content-Le\rngth: 5 + body Hello	context with Content-Length: 5, body read	400
X-Test: ab\rcd	header value abcd	400
GET /c\rd HTTP/1.1	RawUrl is /cd	400
Host: localhost\r\r\n	accepted	400
chunk-size line 1\r0\n	read as a 0x10 byte chunk	400

Both line readers do the same thing. HttpConnection.ReadLine moves to LineState.CR on a CR but keeps appending the bytes that follow, and the next LF ends the line, so a CR anywhere in the request line or a header just disappears. ChunkStream.GetChunkSize behaves the same way. It does have a Missing \n check meant for this case, but it sits after the read loop, and the loop only exits without a break once offset == size, so offset < size is never true there.

RFC 9112 section 2.2 says a recipient of a bare CR must treat the element as invalid or replace the CR with SP. Dropping it does neither, and Content-Le\rngth becoming Content-Length is the same kind of disagreement with an intermediary that #132163 and #132747 closed for whitespace.

The change rejects a CR that isn't immediately followed by a LF in both places: ReadLine throws, which ProcessInput already turns into a 400, and the Missing \n check in GetChunkSize moves into the loop where it can fire. A bare LF is still accepted as a line terminator, and a CR and LF split across two reads still work, since the CR state carries over between calls as before.

On the behavior change: the public API is untouched, and the only requests that now fail are ones that are invalid per the RFC and that http.sys already rejects with a 400 behind the same API, so this brings the managed implementation in line with Windows, as #130910 did for Content-Length.

Tests:

• four bare-CR entries in InvalidClientRequestTests, not gated on the implementation. I replayed exactly those four requests, built the way GetContext_InvalidRequest_DoesNotGetContext builds them, against http.sys on Windows 11 and each got 400 Bad Request; against the shipped managed listener each one got a context.
• Read_ChunkSizeWithBareCR_ThrowsHttpListenerException, managed only, next to the whitespace tests, because http.sys answers 400 before a context is handed out.

I couldn't build the repo on the machines I had (no .NET 11 SDK), so I have not run the xunit tests themselves and am relying on CI for those. What I did run: a net8.0 harness that compiles ChunkStream.cs from the tree as-is and HttpConnection.ReadLine extracted verbatim. At main, 7 of its bare-CR checks fail; with this change they all pass, and the healthy-path checks (CR/LF split across reads, byte-by-byte input, bare LF endings, chunk extensions, several chunks, whitespace in the chunk size still rejected) pass on both sides.

I also noticed that ChunkStream.ReadTrailer never appends the trailer characters themselves to _saved, so trailer fields are dropped, and that 0\r\n\r\r\n leaves the stream waiting for more data. I left that alone here; I can open an issue if it's useful.

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