Auto-enable thin client under gateway mode

[Copilot]

When using gatewayMode(), the SDK should transparently route data-plane requests through the thin client proxy (port 10250) when the account's gateway metadata indicates thin client endpoints are available — with no new public API required.

Changes

• Configs.java: Flipped DEFAULT_THINCLIENT_ENABLED from false → true. Thin client is now opt-out via COSMOS.THINCLIENT_ENABLED=false (system property) or COSMOS_THINCLIENT_ENABLED=false (env var).

• RxDocumentClientImpl.java:

  • Removed the HTTP/2 configuration pre-check from useThinClient. The condition is now simply isThinClientEnabled() && GATEWAY mode. Runtime availability is already gated by useThinClientStoreModel() → hasThinClientReadLocations().
  • Added a dedicated thinClientReactorHttpClient (HTTP/2 force-enabled) created in init() only when useThinClient=true. The existing reactorHttpClient used by the gateway proxy retains HTTP/1.1 — the two are fully separate.
  • thinClientHttpClient() preserves any user-supplied Http2ConnectionConfig pool/stream settings while forcing enabled=true.
  • close() shuts down thinClientReactorHttpClient when it was created.
  • addUserAgentSuffix() now uses the useThinClient instance flag (more accurate than re-reading Configs.isThinClientEnabled()).

• ConfigsTests.java: Updated thinClientEnabledTest — default is now true; test verifies opt-out with false.

Routing behavior

Request type	Connection
Metadata / control plane	Gateway proxy → port 443 (HTTP/1.1)
Data plane (when thin client locations present)	Thin proxy → port 10250 (HTTP/2)
Data plane (no thin client locations in account)	Gateway proxy → port 443 (falls back transparently)

The THINCLIENT_OPT_IN header is sent on the database account metadata request so the gateway returns thin client endpoint locations, which GlobalEndpointManager then uses to set hasThinClientReadLocations.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[FEATURE REQ] [Thin client]: Auto-enable thin client under gateway mode</issue_title>
<issue_description>## 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</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [FEATURE REQ] [Thin client]: Auto-enable thin client under gateway mode #42890

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