Use ast.fix_missing_locations in assertion rewrite

[adamtheturtle]

Summary

• Replace the pure-Python traverse_node location walk with ast.fix_missing_locations after seeding each rewritten statement from the assert.

Timing (pytest's own testing/ suite)

Warm interleaved A/B pytest testing/ --collect-only -q (8 pairs).

	Median
main	0.520s
This change	0.545s
Delta	-0.025s (-4.8%)

Collected 4286 tests each run.

Note: this suite is only ~4k tests / ~0.5–1.5s collect, so single micro-optimizations are often within run-to-run noise.

Test plan

• testing/test_assertion.py

Made with Cursor

[adamtheturtle]

Closing: all changes from this PR were already included in #1 (merged). After rebasing onto main there was nothing left to merge.