Close incomplete multipart upload files

[Yeaseen]

Problem

When a multipart request ends before the current file part is completed,
FormParser.close() closes only the low-level parser. The active File or
decoder-backed writer remains open even though it was never delivered to the
application through on_file.

Completed files are different: once passed to on_file, their lifetime belongs
to the application and closing the parser must not close them.

Solution

Track whether the active multipart file has been transferred to on_file. On
FormParser.close(), close an active writer only when it is still owned by the
parser. Clear the writer after closing it so repeated calls to close() are
safe.

This closes both direct file writers and decoder-backed writers. It preserves
the configured UPLOAD_DELETE_TMP=False behavior: closing an unfinished file
releases its file descriptor but does not unlink the retained temporary file.

Tests

• Verify incomplete binary, Base64, and quoted-printable uploads are closed
  exactly once.
• Verify repeated FormParser.close() calls are safe.
• Verify completed files delivered to on_file remain open and caller-owned.
• Verify UPLOAD_DELETE_TMP=False continues to retain the file after closing.

Verification performed:

163 passed, 3 subtests passed
TOTAL 885 statements, 0 missing, 100% coverage
Ruff: passed
mypy: passed

[codspeed]

Merging this PR will not alter performance

✅ 5 untouched benchmarks
⏩ 5 skipped benchmarks¹

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_{Comparing Yeaseen:fix-close-incomplete-upload (ea974bf) with main (d9cb4c6)}

Footnotes

1. 5 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cubic-dev-ai]

All reported issues were addressed across 2 files

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