quic: add stream.cancel() sending H3_REQUEST_CANCELLED

[trivenay]

Follow-up to #65442. I opened #65509 to finish RFC 9114 compliance for the stream closure error codes, and this implements the cancellation half.

There is currently no way to signal that a request or response was deliberately abandoned: resetStream()/stopSending() default to the no-error code and destroy(error) sends the internal error code. Per RFC 9114 section 4.1.1, a cancelled request should be aborted with H3_REQUEST_CANCELLED.

This adds stream.cancel([reason]), which terminates both directions of the stream using the cancellation code defined by the negotiated application protocol -- H3_REQUEST_CANCELLED (0x10c) for HTTP/3, the no-error code for other applications. The code value is plumbed from the application layer through the session state, same pattern as the request-rejected code in #65442. The existing reset primitives are unchanged.

The main alternative considered in #65509 was exposing the cancellation code as a named constant and letting callers pass it to the existing resetStream()/stopSending(). That keeps the API surface smaller, but cancelling is a single intent that requires aborting both halves of the stream -- a RESET_STREAM and a STOP_SENDING, each carrying the right code for the negotiated protocol. Making every caller do that two-call dance and pick the code is exactly the part that seems worth encapsulating, the same way #65442 picks the rejected code internally. I'm using this PR as a vehicle for that discussion -- happy to change the approach if a different shape is preferred.

The other item from #65509, H3_REQUEST_INCOMPLETE, is still pending: nghttp3 defines the constant but exposes no hook for detecting an incomplete message, so emitting it would need message-completeness tracking on our side. That's considerably more invasive, and honestly I'm not yet convinced the value justifies the complexity -- left out of this PR.

Refs: #65509

[nodejs-github-bot]

Review requested:

• @nodejs/quic

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.99%. Comparing base (666edfd) to head (5a420b1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #66007   +/-   ##
=======================================
  Coverage   89.98%   89.99%           
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  Files         785      785           
  Lines      269282   269308   +26     
  Branches    51296    51292    -4     
=======================================
+ Hits       242323   242357   +34     
+ Misses      17464    17449   -15     
- Partials     9495     9502    +7     

Files with missing lines	Coverage Δ
lib/internal/quic/quic.js	100.00% <100.00%> (ø)
lib/internal/quic/state.js	100.00% <100.00%> (ø)

... and 34 files with indirect coverage changes

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