test: add unit tests for `tracing/`

[KRRT7]

Parent: #2079

Phase 1 — Tracing module

Module	Stmts	Current coverage
tracing/	748	20%

Key files to cover:

• tracing/tracing_new_process.py (508 stmts, 17%)
• tracing/replay_test.py (66 stmts, 12%)
• tracing/pytest_parallelization.py (46 stmts, 0%)
• tracing/tracing_utils.py (64 stmts, 34%)
• tracing/profile_stats.py (64 stmts, 52%)

Test subprocess spawning logic with mocks, profile stats parsing, and parallelization helpers.

Target

100% coverage for tracing/ (~748 statements).