SSE streaming inspection filter

[shaneutt]

Parse Server-Sent Events from inference responses, exposing per-event hooks for inspection and modification by downstream filters.

Requirements

• SSE event parser for chunked transfer-encoded responses
• Per-event filter hooks: inspect, modify, or suppress individual events
• Support for OpenAI-style data: {...} events and data: [DONE] sentinel
• Streaming token extraction from delta content fields
• Event buffering for cross-chunk event reassembly
• Backpressure handling: don't buffer unbounded events

Acceptance Criteria

• Integration test: SSE stream parsed, per-event hook fires for each event
• Token count extracted from streaming deltas matches non-streaming usage response
• Malformed SSE events handled gracefully (logged, forwarded as-is)
• No measurable latency added for pass-through (no-op hook)