http-transport: fix fixed-length response backpressure and abort handling

[denny-il]

Closes #271

What was broken

The fixed-length response path in packages/transports/src/http-server/node.ts registered onWritable for every source chunk. uWS honors only the first registration, so later backpressure could leave a new waiter unresolved while a stale callback retried with the wrong chunk offset.

The fixed-length reader also lacked the abort-side body cancellation used by the chunked path, leaving a stalled reader.read() and its lock alive after client abort.

Fix

• Share one lazily registered writable dispatcher with a swappable pending waiter across fixed-length and chunked response paths.
• Retry each fixed-length chunk from the current uWS write offset after drain events.
• Cancel fixed-length body reads on abort and cancel the source on zero-length or other early response completion.
• Preserve the current shared server-host APIs and transport ownership.

Tests

• Real uWS fixed-length backpressure coverage in packages/transports/tests/neemata/http/node-runtime.spec.ts.
• Deterministic two-chunk partial-write and offset coverage with one writable registration.
• Pending-waiter abort, stalled-read abort, zero-length cancellation, and early-completion source cancellation coverage.

Verification

• pnpm vitest run packages/transports/tests/http-server/fixed-length-stream.spec.ts packages/transports/tests/neemata/http/node-runtime.spec.ts --reporter=agent (11 tests passed)
• pnpm vitest run --project @nmtjs/transports --reporter=agent (239 tests passed)
• pnpm run fmt
• pnpm run check

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📝 Walkthrough

Walkthrough

Node/uWebSockets streaming now shares Promise-based writable waiting across fixed-length and chunked responses. Fixed-length pumping retries partial writes, cancels stalled reads on abort, and cleans up cancellation hooks. Tests cover backpressure, registration reuse, offsets, and abort behavior.

Changes

Writable backpressure coordination

Layer / File(s)	Summary
Shared writable waiter and streaming pump packages/http-transport/src/runtimes/node.ts	Adds centralized writable waiting, integrates it into fixed-length and chunked streaming, retries partial writes, and handles reader cancellation and aborts.
Backpressure and abort coverage packages/http-transport/tests/node-runtime.spec.ts	Adds integration and dispatcher tests for paused reads, resumed writes, single registration reuse, offset tracking, abort cleanup, and fixed-length edge cases.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant ReadableStream
  participant handleFixedLengthStream
  participant HttpResponse
  ReadableStream->>handleFixedLengthStream: read chunk
  handleFixedLengthStream->>HttpResponse: tryEnd chunk
  HttpResponse-->>handleFixedLengthStream: partial write
  handleFixedLengthStream->>HttpResponse: waitWritable
  HttpResponse-->>handleFixedLengthStream: onWritable wake
  handleFixedLengthStream->>HttpResponse: retry remaining bytes

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Possibly related PRs

• neematajs/neemata#267: Both changes modify Node streaming backpressure coordination and writable-event handling.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes implement the shared onWritable dispatcher and fixed-length resume logic needed to address #271.
Out of Scope Changes check	✅ Passed	The test additions and node runtime updates stay focused on the fixed-length response backpressure fix.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main changes to fixed-length response backpressure and abort handling.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch dev/fix-271-fixed-length-backpressure

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[denny-il]

Closing as no longer relevant: the packages this changes are removed in #367 (part of the Effect migration stack).