chore(rust): add ast-grep config and a custom lint rule

Adds sgconfig.yml scoped to litellm-rust plus a rule that flags
format!() passed directly into an error constructor (Error::Variant(...),
PyValueError::new_err(...), etc), since rustfmt wraps that into an
unreadable multi-line call. Local/manual use only for now; not wired
into make lint or CI.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

litellm-rust/ast-grep/rule-tests/no-format-in-error-variant-test.yml

@@ -0,0 +1,20 @@
+id: no-format-in-error-variant
+valid:
+  - |
+    fn f(route: &str) -> Result<(), Error> {
+        let msg = format!("bad route: {route}");
+        Err(Error::InvalidProvider(msg))
+    }
+  - |
+    fn f() -> String {
+        format!("just a string")
+    }
+invalid:
+  - |
+    fn f(route: &str) -> Result<(), Error> {
+        Err(Error::InvalidProvider(format!("bad route: {route}")))
+    }
+  - |
+    fn f(name: &str) -> PyResult<()> {
+        Err(PyValueError::new_err(format!("missing argument: {name}")))
+    }

litellm-rust/ast-grep/rules/no-format-in-error-variant.yml

@@ -0,0 +1,12 @@
+id: no-format-in-error-variant
+language: rust
+severity: warning
+message: >-
+  Don't build an error variant straight from a format!() call; rustfmt wraps it into an
+  unreadable multi-line call. Bind the message to a local first, e.g.
+  `let msg = format!(...); $PATH::$VARIANT(msg)`.
+rule:
+  pattern: $PATH::$VARIANT(format!($$$ARGS))
+constraints:
+  PATH:
+    regex: 'Error$'

litellm-rust/sgconfig.yml

@@ -0,0 +1,4 @@
+ruleDirs:
+  - ast-grep/rules
+testConfigs:
+  - testDir: ast-grep/rule-tests