Motivation

Problem: Terraform AWS provider automatically calls ListTagsForResource after creating resources. When deploying Step Functions with activities, Terraform fails because LocalStack returns ResourceNotFound for activity ARNs, even though the activities were successfully created.

Root Cause: The list_tags_for_resource() function only checked the state_machines store and ignored activities entirely.

Solution: Add tag support for activities following the same pattern used for state machines.

Changes

• Extracted TagManager to a shared module for reuse between activities and state machines
• Added tag_manager field to Activity class with proper initialization
• Updated tag_resource(), untag_resource(), and list_tags_for_resource() to handle both activity and state machine ARNs
• Updated create_activity() to accept and store tags

This implements the TODO comments at lines 1392, 1404, and 1416 in provider.py.

Tests

Added 3 new test methods in test_sfn_api_tagging.py:
✅ test_tag_activity() - Tests tagging activities with various tag lists (empty, single, multiple)
✅ test_untag_activity() - Tests removing tags from activities
✅ test_create_activity_with_tags() - Tests creating activities with initial tags

# Create activity with tags
aws stepfunctions create-activity --name test --tags '[{"key":"env","value":"dev"}]'

# List tags (previously returned ResourceNotFound, now works)
aws stepfunctions list-tags-for-resource --resource-arn <activity-arn>

# Terraform now completes successfully without errors

Related

Closes #13430