Harden chunked encoding parsing

[Youssef1313]

Fixes #66794

• It's easier, IMO, to read the whole updated class compared to reading the diff.
• The removal of the AppContext switch is intentional. It was introduced in older releases servicing, AFAIK, and wasn't intended to ever be in main branch.

[Copilot]

Pull request overview

This PR hardens Kestrel’s HTTP/1.1 chunked transfer-encoding parsing to strictly reject invalid chunk extensions (notably CR/LF inside chunk-ext, including quoted-string cases) per RFC 9110/9112, and updates tests accordingly. It also removes the legacy insecure chunked parsing AppContext switch from the mainline implementation.

Changes:

• Tighten Http1ChunkedEncodingMessageBody chunk-size and chunk-extension parsing (including stricter token/quoted-string validation and CRLF handling).
• Expand/adjust chunked request test coverage for invalid/valid extension forms and boundary splitting.
• Update existing tests’ chunk-extension strings to avoid now-invalid characters (e.g., spaces in tokens).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/Servers/Kestrel/Core/src/Internal/Http/Http1ChunkedEncodingMessageBody.cs	Reworks chunk-size and chunk-ext parsing to be stricter and RFC-aligned (including quoted-string rules).
src/Servers/Kestrel/Core/test/MessageBodyTests.cs	Adds new tests for incomplete chunk-extension/value parsing and max hex digit chunk-size parsing.
src/Servers/Kestrel/test/InMemory.FunctionalTests/ChunkedRequestTests.cs	Expands valid/invalid chunk extension theory cases; updates extension examples; adjusts RemoteExecutor options.
src/Servers/Kestrel/test/InMemory.FunctionalTests/MaxRequestBodySizeTests.cs	Updates chunk-extension strings in payloads to remain valid under stricter parsing.

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