fix: Gemini client support for messages without role (A2A)

[marklysze]

Why are these changes needed?

A2A messages, when converted and sent through to the AG2 Gemini client, do not have role on them. The Gemini client class currently assumes it has role on a message. This handles this by defaulting it to user if not provided.

Related issue number

N/A

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/latest/docs/contributor-guide/documentation/ to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[joggrbot]

📝 Documentation Analysis

All docs are up to date! 🎉

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✅ Latest commit analyzed: 6f0724a | Powered by Joggr

[priyansh4320]

Hi @marklysze , awesome contribution.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
autogen/oai/gemini.py	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
autogen/oai/gemini.py	16.12% <0.00%> (-46.42%)	⬇️

... and 115 files with indirect coverage changes

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