fix(request): require paired abort subscriptions

[ronag]

Summary

• validate that EventTarget-like request signals provide both addEventListener() and removeEventListener()
• validate that EventEmitter-like signals provide on() plus either removeListener() or off()
• support on()/off()-only signals during RequestHandler cleanup instead of later calling a missing removeListener()
• reject half-shaped signals before installing an abort listener that cannot be removed

Validation

• 5 request/signal/retry test files: 52 assertions passed
• ESLint and Prettier pass
• git diff --check passes

[Copilot]

Pull request overview

This PR hardens RequestHandler’s abort-signal handling by ensuring abort listener subscriptions are always removable, preventing leaks and runtime errors when “half-shaped” abort signals are provided.

Changes:

• Validate abort signals as either EventTarget-like (addEventListener + removeEventListener) or EventEmitter-like (on + removeListener/off).
• Ensure cleanup supports on/off-only emitters by using the available removal method during teardown.
• Add tests covering rejection of incomplete signal interfaces and correct listener cleanup for both EventTarget and on/off-only emitters.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
test/request-signal-subscription.js	Adds coverage for rejecting incomplete signals and for proper cleanup with on/off and EventTarget signals.
lib/request.js	Adds signal-shape helpers, tightens validation, and fixes EventEmitter cleanup to use removeListener or off.

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