Fix OpenAI reasoning caps and Gemini transparent images

[pierrebrunelle]

OpenAI agents now send max_completion_tokens by default, so a token cap works with reasoning models such as o3. Explicit max_tokens and max_completion_tokens settings still take precedence, and tool follow-ups continue to omit tools.

Gemini image turns now use PNG with the matching MIME type. Normalizing to RGBA preserves transparency and supports image modes such as CMYK that PNG cannot encode directly.

Regression tests exercise Agent.chat() and Agent.tool_call() through the actual provider SDKs with mocked HTTP transports, inspecting request bodies and decoded image pixels. No live provider calls are required.

Validation:

• Full keyless suite on locked Pixeltable 0.7.17: 103 passed.
• New regression tests on minimum supported Pixeltable 0.7.15: 10 passed.
• Ruff and git diff --check passed.