fix: add validation for empty ias client id

[NicoleMGomes]

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

AgentGatewayClient.get_ias_client_id() previously returned an empty string silently when the IAS clientId was missing from the destination properties (LoB flow) or when client_id was empty in the customer credentials file. Callers had no way to distinguish a valid empty-string result from a misconfiguration, and would typically fail later with an opaque error.

This fix adds explicit validation at both resolution paths:

• LoB flow (get_ias_client_id_lob in _lob.py): raises AgentGatewaySDKError if the clientId property is absent or empty on the IAS destination.
• Customer flow (AgentGatewayClient.get_ias_client_id in agw_client.py): raises AgentGatewaySDKError if credentials.client_id is empty after loading the credentials file.

Tests are updated to assert the new error behaviour, and a new test case covers the empty customer client_id scenario.

Related Issue

Closes #

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. Run the unit tests for the affected classes:

   uv run pytest tests/agentgateway/unit/test_lob.py::TestGetIasClientIdLob tests/agentgateway/unit/test_agw_client.py::TestGetIasClientId -v
   

2. Confirm all 11 tests pass, including:
   • test_raises_when_client_id_property_absent (LoB flow — missing clientId destination property)
   • test_customer_raises_when_client_id_empty (Customer flow — empty client_id in credentials)
3. Confirm that when a valid clientId / client_id is present, the value is returned unchanged.

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

Additional Notes

This is a purely additive validation change. The happy path (non-empty client_id) is unaffected. Any caller that was previously handling an empty string return now needs to handle AgentGatewaySDKError instead — but in practice no legitimate call site would treat an empty client ID as a success.