fix(files): support filename for stream and byte-array multipart uploads

[bsaptarshi]

Summary

Fixes multipart upload behavior for stream/byte file inputs by ensuring a filename is always present.

This addresses #284, where uploads using anonymous bytes/streams (for example, browser recordings) can fail because multipart filename is omitted.

Changes

1) Explicit filename overloads

Added filename-aware overloads in:

• com.openai.models.files.FileCreateParams
• com.openai.models.containers.files.FileCreateParams

New overloads:

• file(InputStream file, String filename)
• file(byte[] file, String filename)

2) Default filename fallback for anonymous uploads

Updated single-argument overloads to provide a safe default filename:

• file(InputStream) now defaults to file.bin
• file(byte[]) now defaults to file.bin

This keeps backward compatibility while improving behavior for anonymous stream/bytes sources.

3) Existing behavior retained

• file(Path) behavior is unchanged and continues to use path.name as filename.
• Multipart content type for binary file parts remains application/octet-stream.

4) Tests

Added/updated tests to verify:

• default filename (file.bin) is applied for single-arg stream/bytes uploads
• explicit filename is preserved when provided
• multipart field content type remains application/octet-stream

Why

file(Path) already included multipart filename metadata, but file(InputStream) and file(byte[]) previously did not.
Some server-side file handling expects filename in multipart parts, causing failures for anonymous stream/bytes uploads.

Validation

Ran targeted tests for modified models:

./gradlew :openai-java-core:test \
  --tests com.openai.models.files.FileCreateParamsTest \
  --tests com.openai.models.containers.files.FileCreateParamsTest

[bsaptarshi]

Hi @TomerAberbach,

Following up on Issue #284. I have added this PR to include default filename fallbacks (file.bin) for InputStream and byte[] overloads, plus new explicit filename overloads.

This should resolve the 400 errors for anonymous uploads while remaining backward compatible. Please help review the changes!

[bsaptarshi]

One more thing to add regarding the test failures. I confirmed they are unrelated to this PR and were already present on upstream/main.

I ran full tests on both:

• upstream/main (without this change)
• this branch (with this change)

The same baseline admin/org failures appear in both runs.
This PR only changes multipart filename handling for stream/byte uploads and related tests.

[bsaptarshi]

Hi @TomerAberbach

Can you or someone take a look at this PR? If it's no.longer required feel free to update the issue and I'll retract this. Not sure what the plan is for this one.