fix(api): snapshot status model override - #7145

Summary

• read system-status default provider and model under one override lock guard
• recover poisoned override state without silently falling back to configured values
• test poisoned override recovery and the no-override configured pair

Verification

• CARGO_INCREMENTAL=0 CARGO_TARGET_DIR=/tmp/librefang-status-model-target cargo test -p librefang-api status_model_snapshot_ --lib
• CARGO_INCREMENTAL=0 CARGO_TARGET_DIR=/tmp/librefang-status-model-target cargo clippy -p librefang-api --all-targets -- -D warnings
• CARGO_INCREMENTAL=0 CARGO_TARGET_DIR=/tmp/librefang-status-model-target cargo check --workspace --lib
• cargo fmt --all -- --check
• git diff --check

Out of scope

• other API default-model consumers
• kernel-level default-model override changes already covered by fix(kernel): clear config override lock poison #7114