bedrock incremental stream path is a buffered-replay (no true token streaming)

[zmaril]

What

The bedrock provider's incremental/streaming path does not stream token-by-token — it produces the full response and replays it as a single buffered result. A caller streaming from a Bedrock model gets the whole message at once instead of incremental updates.

Why

The bedrock Rust driver (crates/pidgin-ai/src/api/bedrock/driver.rs) is buffered-only: it has stream / stream_simple but no stream_streaming / stream_streaming_simple (unlike the anthropic and mistral drivers, which gained true incremental entry points in #343). As a result, BedrockBackend::stream_incremental (crates/pidgin-ai/src/providers/bedrock_backend.rs) delegates to the buffered stream_simple and wraps it via AssistantEventReader::from_buffered.

This is a pre-existing structural limitation, not a regression: before #349 the default Provider::stream_incremental seam impl already did a buffered replay of stream. #349 only made that replay honor reasoning (by replaying stream_simple instead of stream). Reasoning/thinking IS lowered correctly on the bedrock incremental path; the gap is purely that it isn't truly incremental.

pi's bedrock (vendor/pi/packages/ai/src/api/bedrock-converse-stream.ts) uses the real Bedrock ConverseStream API and streams incrementally.

Fix

Port a real incremental entry to the bedrock driver — a stream_streaming (and stream_streaming_simple mirroring the anthropic/mistral shape from #343) backed by the Bedrock ConverseStream API — and route BedrockBackend::stream_incremental through it instead of the from_buffered replay. Then the bedrock incremental tests can assert true event-by-event streaming rather than a buffered replay.

References

• Surfaced in feat(ai): lower reasoning on the bedrock incremental path #349 (bedrock incremental reasoning lowering) and feat(ai): lower reasoning into bedrock requests #342 (bedrock buffered reasoning lowering).
• Incremental streaming pattern to mirror: feat(ai): thread reasoning through the incremental stream seam #343 (anthropic/mistral stream_streaming_simple).