fix: invoke end callback when no chunk is passed

[danielroe]

reproducible in nuxt/image#2094

the ServerResponse never invokes cb when called without a chunk: the callback is detected, the write branch is skipped, writableEnded/writableFinished are set and close/finish emitted, but the promise never settles.

const ipx = createIPX({ storage: ipxFSStorage({ dir: 'some/dir' }) })
await fetchNodeRequestHandler(createIPXNodeHandler(ipx), '/s_300x300/images/colors.jpg') // hangs

this PR calls the callback after finish is emitted, and stops forwarding it to write() so it still fires exactly once (and after finish) when a chunk was passed.

Summary by CodeRabbit

• Bug Fixes
  • Corrected response completion callback timing so callbacks run after the response has finished closing.
  • Ensured callbacks are invoked exactly once, including when ending a response with data, an encoding, or after it has already ended.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 6d3ac68f-0339-43ea-b845-9e5776a495af

📥 Commits

Reviewing files that changed from the base of the PR and between ac5ac4d and 590c1cc.

📒 Files selected for processing (2)

• src/stream/writable.ts
• test/index.test.ts

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📝 Walkthrough

Walkthrough

Changes

Writable end callback behavior

Layer / File(s)	Summary
Callback ordering and validation src/stream/writable.ts, test/index.test.ts	Writable.end invokes its callback after the close and finish events. Tests cover calls with no chunk, chunk data, explicit encoding, and already-ended responses, including single invocation.Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main fix: calling end callbacks when no chunk is provided.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.31%. Comparing base (ac5ac4d) to head (590c1cc).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   51.34%   52.31%   +0.96%     
==========================================
  Files          10       10              
  Lines         409      411       +2     
  Branches       61       62       +1     
==========================================
+ Hits          210      215       +5     
+ Misses        186      184       -2     
+ Partials       13       12       -1     

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