Fix bugs from comprehensive code review (v0.3.1)

[filthyrake]

Summary

Comprehensive bug fix release addressing findings from three independent code reviewers (Gafton, Margo, Scout). Fixes span 8 source files across reliability, correctness, and edge case handling.

HIGH severity fixes

• SSE streaming: Add \n\n delimiters between events (were malformed — missing blank-line separators)
• Streaming errors: Yield SSE error events on backend HTTP failures instead of silently truncating
• vLLM CancelledError: Add missing cancellation handling to streaming path (was leaking connections)
• Backend retry logic: Classify HTTP errors — 4xx = don't retry, 5xx/timeout/connect = retry (was breaking on first failure)
• Graceful startup: Tolerate backend startup failures instead of crashing the app
• temperature_override: Default changed from 0.0 to None so profiles without it passthrough client temperature

MEDIUM severity fixes

• Streaming timeout: Per-chunk 60s timeout wrapper prevents stalled backends from hanging clients forever
• SSE content field: Ensure content present in synthesized SSE delta (OpenAI client compat)
• vLLM assert → RuntimeError: Won't be stripped by python -O
• vLLM schema: arguments typed as string (not nested object) for constrained decoding
• Retry budget: Minimum remaining buffer raised from 10s → 20s
• Rescue regex: Handles quoted strings with parens; tries all bracket groups
• Backend shutdown: 5s timeout prevents hanging during shutdown

LOW severity fixes

• _think_re_cache: Bounded with lru_cache(maxsize=32) instead of unbounded dict
• Escalation summary: Skip appending empty message when best response has valid calls
• Validation: Filter additionalProperties errors from jsonschema (was double-reporting)
• _in_flight counter: Encapsulated in _RequestCounter class with drain event

Version bump

• 0.3.0 → 0.3.1

Test plan

• All 77 existing tests pass
• Deployed to 10.0.0.100 and tested against live Ollama backends
• Proxy starts cleanly, health check passes, profiles loaded

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