tests: Move too_big.json fixture away from the removed Z831/Z931

Jforresterrequested to merge
drop-z831-too-big-fixture into main

The Z831 ("Validate against schema") Function and its built-in Implementation Z931 were dropped in 58e55b6e and 2a9cff4a, but the test_data/function_call/too_big.json fixture still embedded the full Z831 definition twice (as each of two types' Z4K3 validators), leaving it referencing definitions that no longer exist.

This fixture is only consumed by the file-utils tests, which assert purely on byte size, so the embedded function is semantically irrelevant. Swap the dangling identifiers for the still-existing Z866 ("String equality") Function and its built-in Implementation Z966, and update the two argument labels to Z866's own ("first string" / "second string"), for consistency.

Bug: T418886
Assisted-by: Claude Opus 4.8

Merge request reports