test: assert exact invocation count; docs: note against over-defensive checks

Each function is invoked exactly twice, so assert toBe(2) instead of a
loose lower bound. Also add a guideline to AGENTS.md against unnecessary
defensive checks/tests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: lym953

AGENTS.md

@@ -10,6 +10,10 @@ The Datadog Lambda Extension ("Bottlecap") is an AWS Lambda Extension written in
 
 All Rust code lives under `bottlecap/`. Run these from that directory unless noted.
 
+## Code Style
+
+- Do not be overly defensive and add unnecessary checks or tests unless you have a good reason.
+
 ## Pull Requests
 
 - When creating a PR, follow the PR template at `.github/pull_request_template.md`.

integration-tests/tests/durable-cold-start.test.ts

@@ -57,7 +57,7 @@ describe('Durable Function Cold-Start Metric Tag Integration Tests', () => {
       windowStart,
       windowEnd,
     );
-    expect(totalCount).toBeGreaterThanOrEqual(2);
+    expect(totalCount).toBe(2);
     expect(taggedCount).toBe(totalCount);
   });
 
@@ -75,7 +75,7 @@ describe('Durable Function Cold-Start Metric Tag Integration Tests', () => {
       windowStart,
       windowEnd,
     );
-    expect(totalCount).toBeGreaterThanOrEqual(2);
+    expect(totalCount).toBe(2);
     expect(taggedCount).toBe(0);
   });
 });