More reliable built-in Gemini runs

Built-in Gemini over Google's native protocol

Pi now sends built-in Gemini calls through a native gateway route to Vertex AI. The previous OpenAI-compatible route could stall on follow-up calls carrying tool-use data. The native route also preserves the data Gemini needs for those calls. This applies to Agenta's built-in Gemini models, not every connection to Gemini.

Improvements

• Bedrock and SageMaker credentials are passed with each model call. Concurrent calls no longer set one another's AWS credentials through the shared process environment.
• The Python SDK adds a default scheme when AGENTA_API_URL has none, so it can form a valid trace endpoint.

Bug fixes

• A comment on a CSV cell beyond a short header row shows its value without a dangling colon.
• The runner classifies refusals returned by the native Gemini gateway route.