feat: stream SSE tokens in-flight with delta content fallback

[kaissb]

Summary

Zero-copy token accounting for streaming responses: chunks are
processed as they arrive via SSEAccumulator rather than buffering
the full response body.

Changes

• Adds SSEAccumulator in internal/pricing/streaming.go that processes SSE response chunks byte-by-byte as they arrive, building lines across chunk boundaries without buffering the full response body
• Updates budgetTrackingBody in proxy.go to route streaming reads through SSEAccumulator instead of b.buffer, preserving zero-copy semantics while tracking costs in-flight
• Priority chain for token resolution: usage field in final SSE chunk (exact) → sum of choices[0].delta.content bytes (estimated, output only) → body-length estimation fallback (existing)
• Delta-content estimation is significantly more accurate than body-length for SSE responses because it ignores the data:, JSON structure, and newline overhead that inflated previous estimates
• Adds scripts/dev/test-streaming-tokens.sh integration script covering three cases: streaming with usage field, streaming without usage field (delta fallback), and non-streaming baseline

Testing

• go test ./... passes
• go vet ./... passes
• bash scripts/dev/test-streaming-tokens.sh — all 3 tests pass and spent_usd > 0 for each agent
• Unit tests in pricing/streaming_test.go: usage field extraction, delta content fallback, malformed chunks, partial lines spanning chunk boundaries
• Existing pricing_test.go non-streaming path unchanged

Checklist

• PR title follows Conventional Commits format (feat:, fix:, docs:, etc.)
• One logical change per PR
• No new dependencies added (or explicitly approved in description)
• No sensitive data in code or logs
• Doc comments added for new exported symbols