fix: clean up stream abort listeners

[pragnyanramtha]

Summary

• centralize EventStream caller AbortSignal forwarding so listener references are retained
• remove forwarded abort listeners when streams end
• update chat, responses, and assistant stream helpers to use the shared cleanup
• add a ChatCompletionStream regression for listener registration and removal after completion

Why

Stream helpers currently attach anonymous abort listeners to caller-provided signals and never remove them. This can retain timer-backed signals such as AbortSignal.timeout() after the stream completes, similar to the client-level fetchWithTimeout leak but in the helper layer.

Validation

• yarn test tests/lib/ChatCompletionStream.test.ts --runInBand -t 'removes the caller abort listener'
• yarn test tests/lib/ChatCompletionStream.test.ts tests/lib/ResponseStream.test.ts tests/streaming/assistants/assistant.test.ts --runInBand
• ./node_modules/.bin/prettier --check src/lib/EventStream.ts src/lib/AbstractChatCompletionRunner.ts src/lib/AssistantStream.ts src/lib/ChatCompletionStream.ts src/lib/responses/ResponseStream.ts tests/lib/ChatCompletionStream.test.ts
• ./node_modules/.bin/eslint src/lib/EventStream.ts src/lib/AbstractChatCompletionRunner.ts src/lib/AssistantStream.ts src/lib/ChatCompletionStream.ts src/lib/responses/ResponseStream.ts tests/lib/ChatCompletionStream.test.ts
• ./scripts/build
• git diff --check

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