fix(agentgateway): use CLOUD_SDK_CFG_AGW_DEFAULT_* env vars for integration tests

[jeanscherf]

Summary

The Agent Gateway integration tests were reading two non-standard env vars (TENANT_SUBDOMAIN, APPFND_CONHOS_LANDSCAPE) that didn't follow the CLOUD_SDK_CFG_* convention used by every other module. This meant the integration test workflow couldn't pick them up automatically, causing the AGW tests to always fail in CI.

• Rename TENANT_SUBDOMAIN → CLOUD_SDK_CFG_AGW_DEFAULT_TENANT_SUBDOMAIN in the conftest fixture
• Rename APPFND_CONHOS_LANDSCAPE → CLOUD_SDK_CFG_AGW_DEFAULT_LANDSCAPE in the conftest, mapped back to APPFND_CONHOS_LANDSCAPE via os.environ.setdefault (the SDK still reads the original name internally)
• Update docstring and .env_integration_tests.example accordingly
• No workflow changes needed — the existing CLOUD_SDK_CFG_* filter already covers the new names

Test plan

• Trigger Integration Tests workflow manually and verify AGW tests pass

[NicoleMGomes]

Ops, thank you for fixing it