[Files] Transform string into part for metadata in upload_files request

[AMoldova-NI]

• This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

Fix upload_files method to accept metadata field with custom data.

Why should this Pull Request be merged?

This fixes a bug issued here #206

What testing has been done?

Testing done using the script provided in the issue

[Copilot]

Pull request overview

Fixes FileClient.upload_file() multipart encoding so the optional metadata argument is sent as a non-file form part (preventing the File service from rejecting it as a file), addressing the 400 Bad Request described in issue #206.

Changes:

• Encode metadata as a multipart part tuple (None, json, "application/json") instead of passing a raw JSON string to the multipart Part.
• Rename the intermediate variable from metadata_str to metadata_part to reflect the new payload shape.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

tests/integration/file/test_file_client.py:123

• test__upload_get_delete_files__succeeds uploads with cleanup=False, but this test no longer deletes the file. Since the test_file fixture only cleans up IDs recorded when cleanup=True, this will leak a file in the integration environment.

    def test__upload_get_delete_files__succeeds(
        self, client: FileClient, test_file, random_filename_extension
    ):
        # upload a file
        file_id = test_file(file_name=random_filename_extension, cleanup=False)
        assert file_id != ""