chore(examples): use test_key_1 instead of dfx_test_key in encrypted_chat tests

Consistent with the same change made to the backend canister tests in #365.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: marc0olo

examples/encrypted_chat/rust/backend/tests/common/mod.rs

@@ -53,7 +53,7 @@ impl TestEnvironment {
         pic.install_canister(
             canister_id,
             wasm_bytes,
-            encode_one("dfx_test_key").unwrap(),
+            encode_one("test_key_1").unwrap(),
             None,
         );