Antigravity streaming handlers were missing the keepalive mechanism
that exists in the standard gateway, causing proxy/CDN idle timeouts
to break connections during long thinking phases (e.g. claude-opus-4-6).
This resulted in truncated responses with missing tool calls.
Add StreamKeepaliveInterval support to all three Antigravity streaming
paths: Claude SSE, Gemini SSE, and upstream passthrough.
The SSE stream termination marker string was incorrectly included in
DefaultStopSequences, causing Gemini to prematurely stop generating
output whenever the model produced text containing that marker.
The SSE-level protocol filtering in stream_transformer.go already
handles this marker correctly; it should not be a stop sequence for
the model's text generation.
kunish