test: cover extract_linear_layer_generic in aprender-serve/src/safetensors_infer_convert.rs #5534

Test-only ticket, estimated <=200 lines of test. Source: pmat query --coverage-gaps --rank-by impact --exclude-tests on coverage-nightly run 39080471983 (sha e6ab79f; CI runner paths rewritten to this checkout). Impact 2.93.

Function: extract_linear_layer_generic in crates/aprender-serve/src/safetensors_infer_convert.rs. Write tests that assert behaviour (not just no-panic) so the uncovered branches run; per the coverage+contracts rule add or strengthen the matching falsification line in the contract YAML and run pv validate. No unwrap, no new Python.
Before starting: confirm on origin/main the function is still uncovered (pmat query "extract_linear_layer_generic" --coverage --uncovered-only) and that no branch or open PR carries this ticket's number.
Done when: the function's uncovered lines are executed by tests in cargo test -p <crate> --lib and the contract line is in.