fix: avoid dumping gateway config in logs

[lucarlig]

Summary

• remove the startup println! that dumped the full parsed gateway config
• log a structured startup event through tracing after logging initialization
• avoid logging full UserConfig payloads after config lookup; log only subject and virtual-host count
• fix the typo in the configuration retrieval error message touched by this logging path

Why

The gateway should keep useful operational startup and request context without printing entire configuration objects. Full config and user config dumps can expose sensitive operational details or future credential-bearing runtime config.

Scope

This PR intentionally avoids routing, MCP response handling, and dependency-resolution changes. A broader MCP logging pass should be a separate PR with tests if we want to change those call sites.

Validation

• cargo fmt --all --check
• cargo check --locked -p contextforge-gateway-rs-lib -p contextforge-gateway-rs
• cargo clippy --locked -p contextforge-gateway-rs-lib -p contextforge-gateway-rs -- -D warnings

Note: this branch is based on main, so the existing unrelated workspace.dependencies.rmcp.commit warning is still emitted until the dependency-pin PR lands.