Improvement: proxy-sovd - Remove mock path and generate_mock_response_data when SOVD server returns real responses

[arvindr19]

Description

Remove mock gateway path.
generate_mock_response_data and the mock gateway code path exist as an offline stand-in. Once the SOVD server returns real responses these should be removed so the client always talks to a live gateway.

• Remove generate_mock_response_data and related mock branch in SovdClient
• Remove mock_gateway config flag or repurpose it as a test-only feature flag
• Add integration tests against a real/stubbed SOVD endpoint

Additional Context

File: proxy-sovd/src/client.rs
PR: #12