test: add self-hosted zero-config mode tests (#2)

Adds comprehensive tests for self-hosted zero-config mode using WireMock.

Test coverage:
- Client initialization without credentials for localhost
- Gateway Mode with empty/whitespace user tokens
- Proxy Mode without authentication
- Policy enforcement (SQL injection, PII) still works
- Health check without credentials
- First-time user experience simulation
- Verification that auth headers not sent for localhost

Part of Issue #682 (axonflow-enterprise)

Author: saurabhjain1592