`_resolve_max_depth` is a process-wide `lru_cache`, so its once-per-value
warning survives across tests; any earlier in-process resolution of the same
invalid string turned `test_invalid_max_depth_warns_and_preserves_default_capture`'s
"exactly one warning" into 0 (probe: resolve 'nope' first, run the test ->
`assert 0 == 1`). An autouse fixture clears the cache around every test so the
assertions describe the plugin, not the order the tests ran in.
#116713 parsed HERMES_LANGFUSE_MAX_DEPTH inside `_safe_value`, i.e. once per
captured prompt, response, tool input and tool output. With an invalid value
(`abc`) every captured field logged the same "Invalid ... Falling back to 4"
WARNING for the life of the process — one multi-tool turn fills agent.log.
Resolve the depth in `_resolve_max_depth`, an lru_cache keyed on the raw env
string: the warning fires once per distinct bad value, a changed env var is
still picked up by a long-lived process (mirrors `_capture_mode`, which also
reads per call and warns once), and valid values skip the int() parse after
the first call.
Follow-up to #116713 (independent review finding).
