fix: model principal context without api keys

[rpatel-scale]

Summary

• Define AgentexAuthPrincipalContext as a typed base model that omits API-key fields from the object itself.
• Normalize authn responses and cached auth gateway principals through that model before they reach request state.
• Serialize authz principal payloads from the typed model so API keys are not forwarded to authz.
• Add tests asserting api_key/apiKey are absent as object attributes and are not retained in model extras.

Validation

• uv run --project agentex ruff check agentex/src/api/schemas/principal_context.py agentex/src/adapters/authentication/adapter_agentex_authn_proxy.py agentex/src/adapters/authorization/adapter_agentex_authz_proxy.py agentex/src/api/middleware_utils.py agentex/src/api/authentication_middleware.py agentex/src/api/authentication_cache.py agentex/src/domain/services/schedule_service.py agentex/src/domain/use_cases/agent_api_keys_use_case.py agentex/src/domain/use_cases/agents_use_case.py agentex/tests/unit/api/test_principal_context.py agentex/tests/unit/api/test_authentication_cache_metrics.py
• uv run --project agentex --group test pytest agentex/tests/unit/api/test_principal_context.py agentex/tests/unit/api/test_authentication_cache_metrics.py
• uv run --project agentex python - <<'PY' ... runtime check confirmed hasattr(principal, "api_key") == False and model_extra == {}
• git diff --check

Notes

• agentex/tests/integration/api/agents/test_agents_auth_api.py was attempted but did not reach the changed code because local Docker/testcontainers setup could not connect to the Docker socket.