gh-159640: Escape profile data embedded in flamegraph HTML

[zolaski333]

Escape < in serialized profile data before embedding it in flamegraph HTML. This prevents profile strings from closing the script element or starting an HTML comment, while preserving the decoded values.

Add regression coverage for both JSON serialization paths, including mixed-case script tags, Unicode characters, and literal escape sequences.

Validation on Windows:

• All 115 collector tests pass in standard and free-threaded builds.
• Debug tests pass with reference leak checking (-R 3:3).
• patchcheck and git diff --check pass.

Fixes #159640.

• Issue: Sampling profiler flamegraph HTML does not escape </script> in embedded profile data #159640