Improve OpenCode runtime error handling and JSON support detection

[omer-second]

Summary

• Replace the brittle OpenCode run --help probe with a dedicated JSON support detector that caches positive results and reports clearer failure reasons.
• Surface more useful runtime error text from nested and string-shaped CLI error events, with basic secret redaction.
• Add Bedrock auth env key support for OpenCode model detection.
• Extend tests for error extraction, JSON support probing, caching, and Bedrock auth env mapping.

Testing

• Added unit coverage for CLI error extraction and OpenCode JSON support detection.
• Verified the new probe behavior for supported, unsupported, timeout, and cached cases.
• Not run (not requested)