fix(api): cleanly handle abort signals and controller state in SSE stream API

[ArshVermaGit]

Description

This PR addresses flakiness and memory leak issues in the Server-Sent Events (SSE) streaming API (/api/stream), as well as fixing the associated test suite which was failing completely.

Previously, if a client disconnected while the backend was waiting on an asynchronous database poll, the server would subsequently attempt to call controller.enqueue() on an already closed stream. This resulted in noisy TypeError: Invalid state: Controller is already closed exceptions and potential hanging promises.

Additionally, the test/sse-stream-route.test.ts suite was completely failing due to a cross-environment Type mismatch where NextRequest rejected the Node-native AbortSignal constructed by Vitest.

Resolved Issue

Resolves #1794

Changes Made

• Safe SSE Teardown: Introduced a robust isClosed flag to the SSE polling logic. The interval now immediately halts on abort, and any pending async database responses are safely discarded without attempting to enqueue into a dead stream.
• Test Suite Resiliency: Updated the test mocks to bypass strict NextRequest constructor validations by injecting the AbortSignal via Object.defineProperty post-instantiation.

Impact

• Backend Stability: Prevents noisy console errors, resolves race conditions, and ensures clean teardown of resources when users close dashboard tabs.
• Continuous Integration: The SSE test suite is now fully unblocked and reliably passes all concurrency and memory-leak checks.

Testing

• Ran the full SSE test suite locally (npm run test test/sse-stream-route.test.ts).
• Verified that all 15 concurrency and abort cleanup tests pass flawlessly with 0 console errors.

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[ArshVermaGit]

Hi @Priyanshu-byte-coder ! Issue #1794 has been resolved. Please review the PR and merge it under GSSoC. Thanks!