fix(ai): handle escape sequences in INSIDE_OBJECT_KEY state of fixJson

[Ricardo-M-L]

Summary

• The INSIDE_OBJECT_KEY state in fixJson did not handle backslash escape sequences (\\), while the INSIDE_STRING state properly handled them by pushing INSIDE_STRING_ESCAPE onto the stack
• This caused the parser to incorrectly identify the end of an object key when the key contained escape sequences like \", \\, or \n — the escaped quote was treated as the closing delimiter
• Added \\ case to INSIDE_OBJECT_KEY matching the existing INSIDE_STRING behavior, and added test cases covering escaped object keys

Bug

For input like {"key\"name": "value"}, the parser in INSIDE_OBJECT_KEY state encounters \" and treats the escaped " as the end of the key. It then transitions to INSIDE_OBJECT_AFTER_KEY, expecting a : next — but instead finds name, causing incorrect truncation/parsing.

The INSIDE_STRING state (lines 195-214) already handles this correctly:

case '\\': {
  stack.push('INSIDE_STRING_ESCAPE');
  break;
}

This PR adds the same handling to INSIDE_OBJECT_KEY.

Test plan

• Added test: object key with escaped quote (\")
• Added test: object key with escaped backslash (\\)
• Added test: incomplete object with escaped key
• All existing 46 tests continue to pass (49 total with new tests)