fix(openrouter): add defensive JSON parsing and error recovery for streaming responses

- Add attemptJSONParse() function with 4 fallback recovery strategies for truncated JSON
  1. Try parsing as-is
  2. Try trimming trailing whitespace
  3. Try closing incomplete JSON structures ({}, ]}, }]}, [])
  4. Return null if all attempts fail

- Integrate defensive parsing into handleLine() function:
  - Replace simple JSON.parse() with attemptJSONParse() with null checks
  - Add error-like pattern detection to handle error responses with malformed schema
  - Enhanced logging showing raw response preview and recovery attempts
  - Detailed debug logs for truncated/malformed chunks

- Improve handleStreamChunk() robustness:
  - Add defensive check for null/undefined/non-array choices
  - Better logging with diagnostic information for empty choices

This fixes issue #367 where kimi-k2 and similar models that send truncated JSON at newline boundaries would cause silent parsing failures and agent crashes.

Author: ayorindeadunse