test(docs): assert caller-only increment and align counter test style

Add a TXE test proving a different account cannot mutate the owner's
counter, validating the self.msg_sender() ownership fix. Convert
assertions to f-strings to match the canonical counter test, and
sentence-case the tutorial headings.

Author: critesjosh

docs/docs-developers/docs/tutorials/contract_tutorials/counter_contract.md

@@ -1,5 +1,5 @@
 ---
-title: Counter Contract
+title: Counter contract
 description: Code-along tutorial for creating a simple counter contract on Aztec.
 sidebar_position: 0
 references: ["docs/examples/contracts/counter_contract/src/main.nr"]
@@ -152,8 +152,8 @@ aztec codegen -o src/artifacts target
 
 You can now use the artifact and/or the TS class in your Aztec.js!
 
-## Next Steps
+## Next steps
 
-### Optional: Learn more about concepts mentioned here
+### Optional: learn more about concepts mentioned here
 
 - [Functions and annotations like `#[external("private")]`](../../aztec-nr/framework-description/functions/function_transforms.md#private-functions)

docs/examples/contracts/counter_contract_test/src/lib.nr

@@ -17,7 +17,7 @@ unconstrained fn test_initialize_sets_headstart() {
     let (env, contract_address, owner) = setup(headstart);
 
     let value = env.execute_utility(Counter::at(contract_address).get_counter(owner));
-    assert(value == headstart, "initial counter should equal headstart");
+    assert(value == headstart, f"Expected {headstart} but got {value}");
 }
 
 #[test]
@@ -27,6 +27,22 @@ unconstrained fn test_increment_increases_caller_counter() {
 
     env.call_private(owner, Counter::at(contract_address).increment());
 
+    let expected = headstart + 1;
     let value = env.execute_utility(Counter::at(contract_address).get_counter(owner));
-    assert(value == headstart + 1, "counter should be incremented by 1");
+    assert(value == expected, f"Expected {expected} but got {value}");
+}
+
+#[test]
+unconstrained fn test_increment_does_not_affect_other_account() {
+    let headstart = 5;
+    let (mut env, contract_address, owner) = setup(headstart);
+    let other = env.create_light_account();
+
+    env.call_private(other, Counter::at(contract_address).increment());
+
+    let owner_value = env.execute_utility(Counter::at(contract_address).get_counter(owner));
+    assert(
+        owner_value == headstart,
+        f"Expected owner counter unchanged at {headstart} but got {owner_value}",
+    );
 }