Close and unlink multipart temporary files on all exits

[OskarEichler]

Summary

Use block-scoped Tempfile.create for non-chunked multipart encoding so the temporary file is closed and unlinked on encoding, header-write and body-copy errors as well as success.

Reproduction

Create a multipart POST with a fixed boundary and use a BufferedIO whose write method raises IOError. Retain a reference to the generated temporary file and direct TMPDIR to an owned temporary directory. Before, the file remains open and present after the exception; afterward it is closed and removed without waiting for GC.

Verification

• Ruby 4.0.6 through rbenv; existing bundle exec rake test: 201 tests, 914 assertions, zero failures/errors, both baseline and this isolated patch.
• 12 focused checks cover success and separately injected encoding, header-write and body-copy failures, preserving the original outcome and checking immediate file closure/removal with retained references.
• Ruby syntax and git diff --check pass. Supplemental Lint scan retains the same 25 pre-existing findings; no new findings by file/cop.
• No test/spec files were added or modified, per this contribution's explicit no-new-tests constraint. Focused reproductions were run externally.

Compatibility and limitations

No API, wire-format or dependency change. Tempfile.create is available within the existing Ruby >= 2.7 requirement. Verification is on macOS; Windows filesystem semantics were not exercised.

Based on master 23e859e92e10e43cad520fcacac3fd43640dba7b. Optional external OpenSSL tests and other Ruby/OS combinations were not run. Existing related issues/PRs were checked; this is a focused correctness change, not a claim of exhaustive coverage.