fix(core): add truncation recovery to parseSpecStreamLine

[nil957]

Summary

Addresses #196

When LLM responses are truncated mid-stream, parseSpecStreamLine now attempts to repair the JSON by closing unclosed brackets, braces, and strings before giving up.

Problem

During streaming scenarios, network issues or model interruptions can cause incomplete JSON patches like:

{"op":"add","path":"/root","value":"main"

Previously, these would silently fail with no recovery attempt.

Solution

Added attemptJsonRepair() helper that:

1. Tracks unclosed brackets/braces (respecting string boundaries)
2. Closes unclosed strings first
3. Closes remaining brackets/braces in reverse order

Recovery handles:

• Missing closing braces: {"op":"add"... → adds }
• Missing closing brackets: [1,2,3 → adds ]
• Unclosed strings: "value":"hello → adds "
• Nested structures: handles multiple levels

Returns null for truly malformed JSON (mismatched brackets) to avoid false positives.

Testing

Added comprehensive test suite covering:

• Valid JSON parsing (unchanged behavior)
• Truncation recovery scenarios
• Edge cases (malformed vs truncated)

Checklist

• Tests added
• No breaking changes
• Documentation updated (JSDoc comments)

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