[FEATURE REQ] [Thin client]: Auto-enable thin client under gateway mode

[xinlian12]

Summary

When a user calls CosmosClientBuilder.gatewayMode(), the SDK should automatically route through the thin client proxy (port 10250) if the account's gateway metadata response indicates a thin client endpoint is available. No new ConnectionMode enum value is needed — thin client IS gateway mode, just over HTTP/2 to a co-located proxy instead of HTTPS to the gateway frontend.

Expected Behavior

• gateway() mode auto-detects thin client endpoint from gateway metadata response
• If available, routes data plane requests through thin client proxy (port 10250)
• Metadata/control plane requests continue to use port 443
• System property COSMOS.THINCLIENT_ENABLED=false available as opt-out escape hatch
• No new public API surface — transparent to users