rust: Pin aes dependency version

Pin the aes dependency to =0.9.1 in the root Cargo.toml to match the
Fuchsia third_party crate version.

This prevents cargo from resolving to aes 0.9.3 during clean builds,
which requires Rust 1.89 and fails on the current hermetic toolchain.

Test: ./presubmit.sh
Change-Id: If31f68b2b33653b53e791d5db4245cbfb5590033
Reviewed-on: https://bluetooth-review.googlesource.com/c/bluetooth/+/3800

rust/Cargo.toml

diff --git a/rust/Cargo.toml b/rust/Cargo.toml
index 622e389..b0a77c4 100644
--- a/rust/Cargo.toml
+++ b/rust/Cargo.toml
@@ -24,7 +24,7 @@
 
 ## External dependencies
 # Intended to match Fuchsia versions.
-aes = "0.9.1"
+aes = "=0.9.1"
 assert_matches = "1.5.0"
 bitfield = "0.19.4"
 bitflags = "2.11.1"