chore: restore a green make check on main

`make check` currently fails on main before any change is made: one
`cargo fmt --all -- --check` diff and four `cargo clippy --workspace
--all-targets -- -D warnings` errors. All five are pre-existing and
none of them change behaviour.

- `server/tests/knowledge_rules.rs:125` — rustfmt wants the long
  `assert!` split across lines. Applied `cargo fmt --all` verbatim.
- `server/src/plugin/data.rs:206,215` — `path` is only read under
  `#[cfg(unix)]`, so every other target sees an unused binding. Added
  a `#[cfg(not(unix))] { let _ = path; }` arm, matching the
  `let _ = error;` idiom already used at line 189 of the same file.
  Windows behaviour is unchanged: these helpers stay no-ops there.
- `server/src/provider/openai_responses.rs:158` — `collapsible_match`.
  Applied clippy's own suggestion (move `thinking_open` into a match
  guard). The match ends in `_ => {}`, so a failed guard falls through
  to a no-op exactly as the inner `if` did.
- `server/src/store/models.rs:327` — `items_after_test_module`. Moved
  `optional_u64` and `to_i64` above `mod tests`; the bodies are
  untouched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/tests/knowledge_rules.rs

@@ -125,7 +125,10 @@ async fn offline_crud_round_trip_persists_markdown() {
     .unwrap();
     let added: AddResponse = decode(response).await;
     assert!(added.success);
-    assert!(added.id.starts_with("local-"), "offline add uses a local id");
+    assert!(
+        added.id.starts_with("local-"),
+        "offline add uses a local id"
+    );
     let markdown = rules_root.join(format!("{}.md", added.id));
     assert_eq!(
         std::fs::read_to_string(&markdown).unwrap(),