Fix JSON extraction to handle escape sequences during parsing phase

[Copilot]

Addresses feedback on extractServerErrorFromMessage where preprocessing unescaped quotes before brace matching could cause incorrect parsing when error messages contain legitimate \" sequences or braces inside quoted strings.

Changes

• Find all brace positions and try each: Instead of lastIndexOf('{') which can find braces inside string literals, scan all { positions and attempt parsing from each (last to first)

• Track string context during brace matching: Maintain inString and escapeNext state to only count braces outside quoted strings, preventing mismatches when braces appear in message content

• Move unescaping after extraction: Handle doubly-escaped JSON ({\"type\":...}) after finding valid JSON substring, not as preprocessing

• Comprehensive test coverage: Added test suite covering normal/doubly-escaped quotes, nested objects, and braces in string literals

Example error message that now parses correctly:

// Braces in string content no longer interfere
'Error: {"type":"ValidationError","message":"Required fields: {name}, {email}"}'
// → Extracts: "Required fields: {name}, {email}"

// Multiple JSON-like structures - finds the valid one
'First {"invalid":} then {"type":"RealError","message":"Real message"}'  
// → Extracts: "Real message"

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