Canonicalize nested template format specs in annotations (#9076)

Use the existing f-string body unparser for template interpolation format specs, while preserving the outer interpolation source text. Add compile-only Python annotation-constant coverage.

Assisted-by: OpenAI Codex:model identifier unavailable

crates/codegen/src/unparse.rs

@@ -814,7 +814,7 @@ impl<'a, 'b, 'c> Unparser<'a, 'b, 'c> {
 
         if let Some(format_spec) = &interpolation.format_spec {
             self.p(":")?;
-            self.unparse_tstring_body(&format_spec.elements)?;
+            self.unparse_fstring_body(&format_spec.elements)?;
         }
 
         self.p("}")