Update to fastly 13.1.0 and Rust 1.91.1 (#135)

Author: aram356

.tool-versions

@@ -1,3 +1,3 @@
-fasltly 13.0.0
-rust 1.90.0
+fastly 13.1.0
+rust 1.91.1
 nodejs 24.10.0

AGENTS.md

@@ -91,7 +91,7 @@ cargo check
 
 - `fastly.toml`: Fastly service configuration and build settings.
 - `trusted-server.toml`: Application settings (ad servers, KV stores, ID templates).
-- `rust-toolchain.toml`: Pins Rust version to 1.90.0.
+- `rust-toolchain.toml`: Pins Rust version to 1.91.1.
 
 ### Key Functionality Areas