Escape strings in Crashlytics JSON records

[dajiaohuang]

Summary

• Escape quotes, backslashes, and JSON control characters in Crashlytics record keys and values.
• Keep the existing fast path for strings that need no escaping.
• Add buffered and unbuffered writer tests that parse the generated record and verify round-trip values.

Testing

• Added FIRCLSFileTests.testEscapesJSONStrings for quotes, backslashes, newlines, tabs, and a control character.
• Not run locally: Xcode and the Apple toolchain are unavailable in this environment.
• git diff --check passed.

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[paulb777]

Thanks for the contribution! A few housekeeping items:

• CHANGELOG: The entry in Crashlytics/CHANGELOG.md is under # 13.0.0, which has already shipped. Please move it under a # Unreleased heading at the top of the file (add the heading if it isn't there yet).
• PR number: end the entry with this PR's number, e.g. - [fixed] Description. (#16772).
• Formatting: after updating, please run scripts/style.sh so the infra.check job passes. It needs clang-format 23 and swiftformat (see scripts/setup_check.sh). If you can't run it locally, the infra.check log lists the files that need formatting.

[paulb777]

Thanks, this looks correct and async-signal-safe (stack buffers, a static table, no allocation or locks). Note that user-supplied data (custom keys, logs, errors) is already hex-encoded, so the practical effect is on raw strings like thread names, dispatch queue labels and binary image paths. The CHANGELOG entry could say that.

The catalyst failure in CI isn't caused by this PR: it's the FIRCLSContextManagerTests crash fixed in #16776, so merging main should clear it. testEscapesJSONStrings passes.

[paulb777]

Could you add a short comment noting that this path runs from the signal and Mach exception handlers (thread names, queue labels) and must stay async-signal-safe?

[paulb777]

It would be good to also cover FIRCLSFileWriteArrayEntryString (the thread-name path), and a buffered escaped string longer than FIRCLSWriteBufferLength (1000 bytes) so the escaped segments cross a buffer flush.