fix: change system type label

[lentzvitoria]

Disclaimer: Do not include SAP-internal or customer-specific information in this PR (e.g. internal system URLs, customer names, tenant IDs, or confidential configurations). This is a public repository.

Description

Fix label key mismatch in list_active_integrations: the read side was looking up sap-managed-runtime-system-type, but the write side (SPII system handler) stamps fragments with sap-managed-runtime-assigned-application-namespace. As a result, ConnectedSystem.system_type always returned None for every connected system.

The fix aligns _LABEL_SYSTEM_TYPE in _fragments.py with the key actually written to the Destination Service fragment, and updates the corresponding unit test mock to use the correct key.

Related Issue

Closes #<issue_number>

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

How to Test

1. Call agw_client.list_active_integrations() on a tenant with at least one connected backend system.
2. Verify that the returned ConnectedSystem entries have a non-None system_type field (e.g. "sap.pce").
3. Run the unit tests: uv run pytest tests/agentgateway/ -q — all 273 tests should pass.

Checklist

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

Breaking Changes

None. The system_type field was always None before this fix, so populating it correctly is purely additive.

Additional Notes

The label key sap-managed-runtime-assigned-application-namespace is defined as FragmentLabelKey.ASSIGNED_APPLICATION_NAMESPACE in the internal SPII SDK and must stay in sync with _LABEL_SYSTEM_TYPE in this repo. There is no shared contract between the two — if the key is ever renamed on the write side, the read side will silently regress to returning None again.