feat: add reliable streaming with checkpoints, native Responses SSE, …

[gabrion-stewart]

…and typed output

• Introduce application-owned checkpoint lifecycle (CheckpointSink, AgentCheckpoint, CheckpointBoundary) with awaited persistence at request/response/tool-batch/terminal boundaries, including partial streaming snapshots and cancellation/provider-failure records
• Add fallible async ContextPolicy with explicit failure behavior and history processor integration that preserves native tool-call pairs without silent truncation
• Replace buffered Responses streaming fallback with true incremental SSE (responses_stream.rs), adding native terminal metadata, ordered native replay, and strict terminal-status handling
• Add TypedAgentStream and run_stream_typed recovering validator-transformed Output after terminal checkpoint success
• Unify both streaming constructors under a worker supervisor that observes cancellation and receiver closure across the full run lifetime; committed terminal checkpoints win over later detach/cancel
• Harden OpenAI Chat SSE: byte-buffered UTF-8 parsing, single terminal event, deterministic part closing, tool-call validation before dispatch, and premature-EOF errors
• Run configured schema parsing, async output validators, and dynamic prompts in streaming; validation exhaustion fails closed instead of signaling valid output
• Honor streaming parallel-tool settings with bounded, ordered results and retries; correct explicit output-tool mixed-call Early/Exhaustive decisions in both execution paths
• Redact sensitive Debug output for thinking deltas, model responses, and native terminal metadata