fix(integration-tests): make MyFirstApiKey value stack-scoped

[licjun]

Problem

The api_with_authorizer_apikey integration test template hardcodes both the AWS::ApiGateway::ApiKey.Value and the Lambda authorizer's usageIdentifierKey to the literal "needatleast20characters".

AWS API Gateway enforces an undocumented uniqueness constraint on value across account + region (returns 409 ConflictException / HandlerErrorCode: AlreadyExists on collision). Any leftover key from a previous run — for example, a test runner that timed out before cleanup could complete — will collide with the next run's stack creation, and the stack rolls back to ROLLBACK_COMPLETE. Both the first attempt and any --reruns retry hit the same collision (same hardcoded literal), so the test fails deterministically until the orphaned key is deleted externally.

Fix

Substitute ${AWS::StackName} into both Value and the authorizer's usageIdentifierKey. Because ${AWS::StackName} is unique per CFN stack (which is unique per run), the API Key value is unique per run, and the authorizer + key still resolve to the same string at runtime.

Length bounds (empirically verified)

• CreateApiKey.value min = 20 characters ("API Key value should be at least 20 characters")
• CreateApiKey.value max = 128 characters ("API Key value exceeds maximum size of 128 characters")
• Substituted length is 19 (prefix "needatleast20chars-") + len(stackName). For representative integration-test stack names (~60–70 characters), the result is well within [20, 128].

Testing

Verified end-to-end in a dev account:

• Deployed the fixed template twice with different stack names (fix-test-stack-alpha and fix-test-stack-beta) — both CREATE_COMPLETE, no AlreadyExists collision (previously, one of the two would fail with this signature).
• Confirmed each stack's API Key value equals needatleast20chars-<stackName> (i.e., ${AWS::StackName} is correctly substituted at deploy time).
• Confirmed runtime behavior: GET /lambda-token-api-key with Authorization: allow returns 200 (authorizer's usageIdentifierKey matches the API Key value → usage plan matches), and Authorization: deny returns 403.
• The corresponding test (integration/combination/test_api_with_authorizer_apikey.py) reads the API Key value from AWS at runtime (get_api_key(..., includeValue=True)) and does not depend on any specific literal, so no test-code changes are needed.