TransferEncoding chunked testing only if it is last header value

[rokonec]

Fixes #63053

Context

Transfer-Encodings are ordered and so the check should only succeed if "chunked" is the final Transfer-Encoding value
See #63053

Changes made

Test only final value.
During TransferEncodingChunked is set to true, it is ensured chunked will be last value, even when it was already present.

Testing

Few new unit tests.

Note

This could be a breaking change if server behave out of spec. Currently if server send Transfer-Encoding: chunked, gzip, but actually do gzip and then chunk, client would have handled it without error, with changes in this PR it will fail to receive content.

[MihaZupan]

This is also changing how we interpret response bodies that contain "chunked" as the non-last value.
By the spec, we should likely be falling back to reading until the connection is closed instead of using the content length value.

---

We rely on headers.TransferEncodingChunked to decide whether to apply chunking when writing request bodies (e.g. in WinHttpHandler or SocketsHttpHandler's HttpConnection.cs).

Instead of potentially generating garbage, should we tighten request validation to throw if a Transfer-Encoding header was set but chunked wasn't the last value?

---

Is this part in H3 correct?
https://datatracker.ietf.org/doc/html/rfc9114#section-4.1-11

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3RequestStream.cs

Lines 653 to 657 in e60db07

	// H3 does not support Transfer-Encoding: chunked.
	if (request.HasHeaders && request.Headers.TransferEncodingChunked == true)
	{
	request.Headers.TransferEncodingChunked = false;
	}

Should we unconditionally remove the header instead?

---

Our docs currently state:

Gets or sets a value that indicates if the Transfer-Encoding header for an HTTP request contains chunked.

This feels like it has a non-zero risk of breaking someone. Should we push it to .NET 10 just in case?

[MihaZupan]

This will now do an extra pass over the values in the common case where the value wasn't set yet.

Can we instead do something like

_parent.RemoveParsedValue(KnownHeaders.TransferEncoding.Descriptor, HeaderUtilities.TransferEncodingChunked);
_parent.AddParsedValue(KnownHeaders.TransferEncoding.Descriptor, HeaderUtilities.TransferEncodingChunked);

[MihaZupan]

Suggested change

	if (parent.ContainsParsedValue(KnownHeaders.TransferEncoding.Descriptor, HeaderUtilities.TransferEncodingChunked, lastValueOnly:true))
	if (parent.ContainsParsedValue(KnownHeaders.TransferEncoding.Descriptor, HeaderUtilities.TransferEncodingChunked, lastValueOnly: true))

[rokonec]

I believe this PR shall be abandoned.
Reasoning: #63053 (comment)