fix(api): recover message model override - #7144

Summary

• route the agent message provider preflight through a poison-recovering default-model override reader
• preserve provider, model, and API-key environment state while logging recovery and clearing poison
• add a held-write-lock panic regression that verifies preserved fields and later ordinary reads and writes

Verification

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

Out of scope

• kernel override accesses covered by fix(kernel): clear config override lock poison #7114
• other API default-model consumers and provider mutation routes