fix(WS2001): handle intrinsic functions in AccessLogSetting.Format

[abhu85]

Summary

Fixes #253

When AccessLogSetting.Format (REST APIs) or AccessLogSettings.Format (HTTP APIs) is set to an intrinsic function like !Ref or !Sub, the WS2001 rule crashes with:

E0002 Unknown exception while processing rule WS2001: "'dict_node' object has no attribute 'strip'"

Root cause: The _check_log_format method in api_gateway.py calls .strip() on line 78 without checking if the input is a string. When CloudFormation intrinsic functions are used, the value is a dict-like node (e.g., {"Ref": "LogFormat"}), not a string.

Fix: Added a type check at the start of _check_log_format to skip validation when the format is not a string (i.e., when it's an intrinsic function). This prevents the crash and allows templates with parameterized log formats to pass linting.

Changes

• Added isinstance(log_format, str) check in _check_log_format method
• Added test templates for both REST APIs (!Ref) and HTTP APIs (!Sub)

Test Plan

• Reproduced the bug with the exact template from the issue
• Verified fix prevents the crash
• All 72 existing tests pass
• New test templates added:
  • ws2001-rest-ref-format.pass.yaml - Tests !Ref with REST API
  • ws2001-http-sub-format.pass.yaml - Tests !Sub with HTTP API
• make pr passes (linting, security, complexity checks)

Commands run:

# Reproduce bug (before fix)
cfn-lint template.yaml -a cfn_lint_serverless.rules
# E0002 Unknown exception while processing rule WS2001: "'dict_node' object has no attribute 'strip'"

# After fix - no errors
cfn-lint template.yaml -a cfn_lint_serverless.rules
# (no output = success)

# Run tests
pytest tests/ -v
# 72 passed

# Run PR checks
make pr
# All checks pass

Generated with Claude Code

[abhu85]

Friendly ping for review when you have a chance. This fixes the crash reported in #253 when using intrinsic functions like !Ref or !Sub in AccessLogSetting.Format. All tests pass and make pr checks are green.