fix(aws-lambda): fix flake in aws lambda tests

### Description

- Catch `subprocess.TimeoutExpired` during SAM teardown and force-kill,
so slow shutdown cannot fail the entire test suite.
- Added temporary flake-check workflow that runs each test aws-lambda
test 5 times to verify this removes the flake (workflow is removed in
the last commit; was just to verify that it works).
[https://github.com/getsentry/sentry-python/actions/runs/30539137588/job/90862136512?pr=6937](last
successful run)
- Switched SAM `--warm-containers` startup from `EAGER` to `LAZY` so
Docker containers start on first invocation instead of all 21 at SAM
startup. Verified locally by running
`TESTPATH=tests/integrations/aws_lambda/test_aws_lambda.py uv run tox -e
py3.8-aws_lambda ` both for `EAGER` & `LAZY` 10x independently:
```
EAGER:
=================================================
Metric              |   Mean (s) |  Std Dev (s)
=================================================
Setup time          |      43.69 |         3.99
First call time     |       1.92 |         0.22
Max Teardown time   |       7.49 |         0.27
Total run time      |     108.53 |         4.56
=================================================
LAZY:
=================================================
Metric              |   Mean (s) |  Std Dev (s)
=================================================
Setup time          |      33.32 |         1.82
First call time     |       6.32 |         0.63
Max Teardown time   |       6.90 |         0.18
Total run time      |     102.06 |         3.60
=================================================
```

Resolves: #6911 & PY-2632

Author: pabloDeputter

tests/integrations/aws_lambda/test_aws_lambda.py

@@ -14,6 +14,7 @@
 
 DOCKER_NETWORK_NAME = "lambda-test-network"
 SAM_TEMPLATE_FILE = "sam.template.yaml"
+SAM_SHUTDOWN_TIMEOUT = 10
 
 
 @pytest.fixture(scope="session", autouse=True)
@@ -54,7 +55,7 @@ def test_environment():
             "--template",
             SAM_TEMPLATE_FILE,
             "--warm-containers",
-            "EAGER",
+            "LAZY",  # Start each Docker container on its function's first invocation
             "--docker-network",
             DOCKER_NETWORK_NAME,
         ],
@@ -78,13 +79,13 @@ def before_test():
 
     finally:
         print("[test_environment fixture] Tearing down AWS Lambda test infrastructure")
-
         process.terminate()
-        process.wait(timeout=10)  # Give it time to shut down gracefully
-
-        # Force kill if still running
-        if process.poll() is None:
+        try:
+            # Teardown is typically ~7s; escalate with kill if SAM exceeds this.
+            process.wait(timeout=SAM_SHUTDOWN_TIMEOUT)
+        except subprocess.TimeoutExpired:
             process.kill()
+            process.wait()
 
 
 @pytest.fixture(autouse=True)