Return 500 for invalid transfer encoding on non-body responses

[zeohhh]

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Return a 500 response instead of terminating the connection when an
application completes with an invalid Transfer-Encoding header on a
non-body response.

Description

HttpProtocol.ProduceStart marked the response as HeadersCommitted
before calling CreateResponseHeaders.

When header validation detected an invalid Transfer-Encoding after the
application had completed, Kestrel attempted to replace the response with
a 500 response. This failed because the response was already considered
started, causing the connection to terminate without sending a response.

This change moves the transition to HeadersCommitted until after response
header creation and validation succeeds, while keeping it before
Output.WriteResponseHeaders.

A functional regression test covers GET responses with status codes 204,
205, and 304, as well as HEAD responses. Existing explicit response-start
behavior remains unchanged.

Testing

• Passed all tests under src/Servers/Kestrel/test.
• Verified all four regression test cases pass.

Fixes #4382

[zeohhh]

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