fix(clippy): clear -D warnings across changed code (#2599)

Resolves the workspace clippy warnings so the release gate
(`cargo clippy --workspace --all-targets -- -D warnings`) is clean:

- chat.rs: elide needless lifetime on `next_arcee_waf_trigger` (returned
  strs are `'static`).
- llm_client/mod.rs: use `enumerate()` instead of a manual loop counter in
  `truncate_for_error` (explicit_counter_loop).
- ui.rs: `#[allow(clippy::too_many_arguments)]` on `apply_model_picker_choice`
  with rationale (8 distinct handles/states; a struct would only obscure it).
- file_picker.rs: gate the test-only `WALK_DEPTH` const and `new_with_relevance`
  convenience ctor behind `#[cfg(test)]` (the #2488 change moved production
  callers to `new_with_relevance_and_depth`).

The 6 schema_migration registry structs the issue noted are no longer flagged
(their trait impls keep them live). Also normalizes rustfmt formatting on the
touched lines.

https://claude.ai/code/session_01MQrnh6wHfrEYN5BBdMarC1

crates/tui/src/client/chat.rs

@@ -618,10 +618,7 @@ fn arcee_waf_safe_text_parts(content: &str) -> Option<Vec<Value>> {
     split_any.then_some(parts)
 }
 
-fn next_arcee_waf_trigger<'a>(
-    content: &'a str,
-    cursor: usize,
-) -> Option<(usize, &'a str, &'a str, &'a str)> {
+fn next_arcee_waf_trigger(content: &str, cursor: usize) -> Option<(usize, &str, &str, &str)> {
     ARCEE_WAF_TEXT_SPLIT_TRIGGERS
         .iter()
         .filter_map(|(trigger, left, right)| {

crates/tui/src/config.rs

@@ -5069,7 +5069,8 @@ mod tests {
     fn warns_when_allow_shell_nested_under_general_section() {
         // #2589: the reporter's config nested top-level keys under sections that
         // do not exist, so they were silently dropped and shell tools vanished.
-        let raw = "[general]\nallow_shell = true\n\n[sandbox]\nsandbox_mode = \"danger-full-access\"\n";
+        let raw =
+            "[general]\nallow_shell = true\n\n[sandbox]\nsandbox_mode = \"danger-full-access\"\n";
         let warning =
             warn_on_misplaced_top_level_keys(raw).expect("misplaced keys should produce a warning");
         assert!(warning.contains("general.allow_shell"));

crates/tui/src/llm_client/mod.rs

@@ -456,14 +456,12 @@ fn collapse_whitespace(input: &str) -> String {
 
 fn truncate_for_error(input: &str, max_chars: usize) -> String {
     let mut out = String::with_capacity(input.len().min(max_chars + 32));
-    let mut count = 0usize;
-    for ch in input.chars() {
+    for (count, ch) in input.chars().enumerate() {
         if count >= max_chars {
             out.push_str("...");
             return out;
         }
         out.push(ch);
-        count += 1;
     }
     out
 }

crates/tui/src/tui/file_picker.rs

@@ -32,9 +32,11 @@ use crate::workspace_discovery::{DISCOVERY_ALWAYS_DIRS, path_is_excluded_from_di
 /// equivalent overlay.
 const MAX_CANDIDATES: usize = 20_000;
 
-/// Default walk depth for the initial scan when no explicit depth is supplied.
-/// Mirrors the `Workspace` fuzzy index default (`DEFAULT_COMPLETIONS_WALK_DEPTH`).
-/// `mention_walk_depth = 0` overrides this with an unlimited walk.
+/// Default walk depth used by the picker's own tests. Production callers pass
+/// the configured `mention_walk_depth` (default 10, `0` = unlimited) through
+/// [`FilePickerView::new_with_relevance_and_depth`], mirroring the `Workspace`
+/// fuzzy index default (`DEFAULT_COMPLETIONS_WALK_DEPTH`).
+#[cfg(test)]
 const WALK_DEPTH: usize = 10;
 
 /// Visible candidate rows in the overlay.
@@ -125,7 +127,10 @@ pub struct FilePickerView {
 
 impl FilePickerView {
     /// Build a picker with working-set relevance hints, using the default
-    /// walk depth ([`WALK_DEPTH`]).
+    /// walk depth ([`WALK_DEPTH`]). Test-only convenience; production code uses
+    /// [`FilePickerView::new_with_relevance_and_depth`] with the configured
+    /// `mention_walk_depth`.
+    #[cfg(test)]
     pub fn new_with_relevance(workspace_root: &Path, relevance: FilePickerRelevance) -> Self {
         Self::new_with_relevance_and_depth(workspace_root, relevance, WALK_DEPTH)
     }

crates/tui/src/tui/ui.rs

@@ -4985,6 +4985,10 @@ async fn drain_web_config_events(
 /// `~/.deepseek/settings.toml` so it survives a restart, push the change to
 /// the running engine via `Op::SetModel`/`Op::SetCompaction`, and surface
 /// a one-line status describing what changed.
+// The model/effort transition needs both the previous and next model+effort
+// plus the engine, app, and config handles; bundling them into a struct here
+// would only obscure a straightforward orchestration step.
+#[allow(clippy::too_many_arguments)]
 async fn apply_model_picker_choice(
     app: &mut App,
     engine_handle: &mut EngineHandle,