Fix thin-client routing fallback endpoint

[arnabnandy7]

Description

Fixes #49299.

This PR fixes a Cosmos thin-client routing fallback issue where ThinClientStoreModel could resolve to a RegionalRoutingContext with a gateway endpoint but no thin-client endpoint, causing rootUri to be null.

Changes include:

• Populate the default regional routing context with the matching thin-client endpoint when available.
• Remap gateway-only routing contexts to their thin-client-capable equivalent during thin-client request resolution.
• Add a defensive error in ThinClientStoreModel when a thin-client endpoint is unexpectedly unavailable.
• Add regression tests for default regional endpoint fallback and explicit gateway-only routing context resolution.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message.

Testing Guidelines

• Pull request includes test coverage for the included changes.

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[Copilot]

Pull request overview

This PR addresses a Cosmos thin-client routing fallback bug where ThinClientStoreModel could end up with a RegionalRoutingContext that has a gateway endpoint but no thin-client endpoint, leading to rootUri being null (reported in #49299). The changes focus on ensuring thin-client routing contexts are properly populated/remapped during endpoint resolution and adding regression coverage.

Changes:

• Add a defensive failure in ThinClientStoreModel.getRootUri when the resolved routing context has no thin-client endpoint.
• Enhance LocationCache.resolveServiceEndpoint to remap gateway-only routing contexts to thin-client-capable equivalents when thin-client mode is enabled, and attempt to populate the default routing context’s thin-client endpoint during account location processing.
• Add unit tests covering default routing context thin-client endpoint population and explicit gateway-only routing context remapping.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
sdk/cosmos/azure-cosmos/src/main/java/com/azure/cosmos/implementation/ThinClientStoreModel.java	Adds a defensive error when a thin-client endpoint is unexpectedly missing during thin-client request routing.
sdk/cosmos/azure-cosmos/src/main/java/com/azure/cosmos/implementation/routing/LocationCache.java	Adds thin-client-aware routing context resolution/remapping and attempts to populate thin-client endpoints on the default routing context.
sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/implementation/routing/LocationCacheTest.java	Adds unit tests intended to prevent regressions in thin-client endpoint fallback/remapping behavior.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.