fix(assets): serve the chevron this repo draws, not the stock one

`assets/icons/chevrons-up-down.svg` is drawn to the weight every other
glyph in that directory uses — `stroke-width="2.1"`, all twenty of them —
but it was the one path missing from `agent_icon`, so it fell through to
`gpui_component_assets` and the tab strip drew the stock 2.0 chevron
beside neighbours a hair heavier. Nothing failed, which is why nothing
said so.

`every_git_icon_resolves` could not catch it: it walks three paths
written out by hand, and its own comment names this exact risk — "an SVG
on disk that nobody added to the match above silently renders as
nothing". A list cannot notice the file it was never told about.

So the new test reads `assets/icons` instead and requires every `.svg` in
it to come back from `agent_icon`. Confirmed it fails without the arm
this commit adds, naming the file.

Found by checking shipped data rather than code — the same pass that
turned up the newline descriptions in `assets/completions`.

src/ui/assets.rs

@@ -47,6 +47,9 @@ fn agent_icon(path: &str) -> Option<&'static [u8]> {
         "icons/machine-local.svg" => include_bytes!("../../assets/icons/machine-local.svg"),
         "icons/machine-remote.svg" => include_bytes!("../../assets/icons/machine-remote.svg"),
         "icons/refresh.svg" => include_bytes!("../../assets/icons/refresh.svg"),
+        "icons/chevrons-up-down.svg" => {
+            include_bytes!("../../assets/icons/chevrons-up-down.svg")
+        }
         "icons/agents/claude.svg" => include_bytes!("../../assets/icons/agents/claude.svg"),
         "icons/agents/codex.svg" => include_bytes!("../../assets/icons/agents/codex.svg"),
         "icons/agents/gemini.svg" => include_bytes!("../../assets/icons/agents/gemini.svg"),
@@ -98,6 +101,33 @@ mod tests {
         }
     }
 
+    /// Reads the directory rather than a list, because a list is what let
+    /// `chevrons-up-down.svg` sit here unserved: it was drawn to this repo's
+    /// weight like every other glyph in `assets/icons`, and being absent from
+    /// the match it fell through to the stock one, a hair lighter than its
+    /// neighbours in the same row. Nothing failed, so nothing said so.
+    #[test]
+    fn every_icon_this_repo_ships_is_the_one_that_draws() {
+        let dir = concat!(env!("CARGO_MANIFEST_DIR"), "/assets/icons");
+        let mut checked = 0;
+        for entry in std::fs::read_dir(dir).expect("assets/icons is readable") {
+            let entry = entry.expect("a readable directory entry");
+            let name = entry.file_name();
+            let name = name.to_string_lossy();
+            if !name.ends_with(".svg") {
+                continue;
+            }
+            let path = format!("icons/{name}");
+            assert!(
+                agent_icon(&path).is_some(),
+                "assets/icons/{name} ships but nothing serves it, so the stock \
+                 glyph draws in its place"
+            );
+            checked += 1;
+        }
+        assert!(checked > 15, "only {checked} icons were checked");
+    }
+
     #[test]
     fn every_git_icon_resolves() {
         // An SVG on disk that nobody added to the match above silently renders