fix: avoid setting Part.partMetadata when metadata is empty

[hemasekhar-p]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: PartConverter.toGenaiPart() unconditionally sets Part.Builder#partMetadata(), which the GenAI SDK rejects on Vertex AI / Gemini Enterprise Agent Platform #1421

Problem:
In PartConverter.java, toGenaiPart() and convertFilePartToGenAiPart() unconditionally set Part.Builder#partMetadata(...) whenever the A2A part's metadata map was non-null (textPart.getMetadata() != null and filePart.getMetadata() != null). Because PartConverter.fromGenaiPart() initializes A2A parts with default empty metadata maps ({}), any round-tripped or converted A2A message attached empty metadata {} to Part#partMetadata.

Solution:

1. Updated PartConverter#toGenaiPart (for TextPart) to only call partBuilder.partMetadata(...) when textPart.getMetadata() != null && !textPart.getMetadata().isEmpty().
2. Updated PartConverter#convertFilePartToGenAiPart (for FileWithUri and FileWithBytes) to check metadata != null && !metadata.isEmpty() before calling builder.partMetadata(metadata).
3. Added unit tests in PartConverterTest to verify that TextPart and FilePart with empty metadata maps do not set Part.partMetadata.

Testing Plan

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Unit Tests:

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