Module stream 

Real streaming adapter for Bedrock’s /converse-stream endpoint.

Parses AWS eventstream frames into StreamChunks so callers see text deltas and tool-call argument deltas as they arrive, rather than waiting for the full response.

Bedrock event types handled:

• messageStart → ignored (role is always assistant)
• contentBlockStart → StreamChunk::ToolCallStart (only for tool blocks)
• contentBlockDelta → StreamChunk::Text or StreamChunk::ToolCallDelta
• contentBlockStop → StreamChunk::ToolCallEnd (only for open tool blocks)
• messageStop → ignored (finish reason is carried by Done)
• metadata → captured into usage, emitted at Done
• exception / error → StreamChunk::Error