fix: bound JSON nesting depth to prevent stack overflow (#540)

[slsgzs-cloud]

Summary

Two fixes in this PR:

Fix 1: Bound JSON nesting depth to prevent stack overflow (#540)

parse_value recursively calls parse_array/parse_object without tracking a depth limit. Nested metadata can consume call stack proportional to input nesting.

• Add MAX_DEPTH = 256 constant and depth: usize field to Parser
• Check depth >= MAX_DEPTH at the start of parse_array and parse_object
• Return Err("maximum nesting depth exceeded") when limit reached

Fix 2: Stop classifying valid program stderr as compiler failure (#538)

run_and_classify checked looks_like_fail(result.stderr) before comparing the expected exit status. Any stderr containing phrases such as stack overflow, SyntaxError, or error[E was rejected, even when the program deliberately printed that text and exited with the expected status.

Fix: check exit code first. If the program exits with the expected status, it succeeded — don't reject based on stderr content alone. The compiler/child output is combined, so a real compiler failure would show as a non-zero exit code, not just stderr text.

[LunaStev]

The implementation direction looks good, but both #538 and #540 explicitly require regression coverage.

Could you add tests for the acceptance cases before we merge this?

For #540, please cover array/object/alternating nesting around the chosen depth limit and verify overly deep input returns an error rather than crashing.

For #538, please cover successful stderr containing the failure sentinels, expected non-zero exits, and a genuine compiler failure so we know removing the stderr heuristic does not hide producer failures.