fix: harden API macro expansion hygiene

crates/api-macros/tests/ui/marker_collision.rs

@@ -0,0 +1,14 @@
+use api_macros::api;
+
+pub struct Response;
+
+#[api]
+pub trait MarkerCollision {
+    #[get("/first")]
+    async fn foo_bar(&self) -> Response;
+
+    #[get("/second")]
+    async fn foo__bar(&self) -> Response;
+}
+
+fn main() {}

crates/api-macros/tests/ui/marker_collision.stderr

@@ -0,0 +1,11 @@
+error: generated operation marker `FooBar` collides with method `foo_bar`
+  --> tests/ui/marker_collision.rs:11:14
+   |
+11 |     async fn foo__bar(&self) -> Response;
+   |              ^^^^^^^^
+
+error: first method generating marker `FooBar`
+ --> tests/ui/marker_collision.rs:8:14
+  |
+8 |     async fn foo_bar(&self) -> Response;
+  |              ^^^^^^^