Make Cosmos DB tests read COSMOSDB_ENDPOINT and COSMOSDB_KEY from environment variables

[Copilot]

Motivation and Context

The CI workflow sets COSMOSDB_ENDPOINT and COSMOSDB_KEY environment variables for Cosmos DB emulator tests, but both test classes ignored them entirely — using hardcoded private const string values instead. The workflow configuration was dead.

Description

• Changed EmulatorEndpoint and EmulatorKey from private const string to private static readonly string in both CosmosCheckpointStoreTests and CosmosChatHistoryProviderTests, reading from env vars with the original hardcoded values as fallbacks.
• Renamed fields to follow the codebase s_ prefix convention for static fields (s_emulatorEndpoint, s_emulatorKey).

// Before
private const string EmulatorEndpoint = "https://localhost:8081";
private const string EmulatorKey = "C2y6yDjf5/...";

// After
private static readonly string s_emulatorEndpoint = Environment.GetEnvironmentVariable("COSMOSDB_ENDPOINT") ?? "https://localhost:8081";
private static readonly string s_emulatorKey = Environment.GetEnvironmentVariable("COSMOSDB_KEY") ?? "C2y6yDjf5/...";

No behavioral change locally; in CI the workflow-supplied values now take effect.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? No.

[Copilot]

Pull request overview

This PR enables Cosmos DB tests to respect environment variables set by the CI workflow, which were previously ignored due to hardcoded connection settings. The changes align the test classes with the CI configuration while maintaining backward compatibility for local development.

Changes:

• Converted hardcoded Cosmos DB connection constants to read from COSMOSDB_ENDPOINT and COSMOSDB_KEY environment variables with fallback to original default values
• Renamed fields from EmulatorEndpoint/EmulatorKey to s_emulatorEndpoint/s_emulatorKey following the codebase's static field naming convention

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
dotnet/tests/Microsoft.Agents.AI.CosmosNoSql.UnitTests/CosmosCheckpointStoreTests.cs	Updated to read Cosmos DB connection settings from environment variables, renamed fields to follow s_ prefix convention, updated all 2 usages
dotnet/tests/Microsoft.Agents.AI.CosmosNoSql.UnitTests/CosmosChatHistoryProviderTests.cs	Updated to read Cosmos DB connection settings from environment variables, renamed fields to follow s_ prefix convention, updated all 4 usages