Migrate region discovery to IMDS /compute JSON endpoint (#6039)

[Robbie-Microsoft]

Summary

Implements #6039 per the merged spec docs/region_discovery_imds_compute.md (spec PR #6044).

Migrates MSAL.NET region auto-discovery from the legacy IMDS text endpoint /metadata/instance/compute/location (api-version=2020-06-01, format=text) to the newer JSON endpoint /metadata/instance/compute (api-version=2021-02-01), parsing the location field.

There is no public API change, no telemetry schema/string change, and no feature flag. REGION_NAME precedence, process-wide caching, retry, timeouts, and the api-version 400 fallback are all preserved.

What changed

• RegionManager.cs: ImdsEndpoint -> /metadata/instance/compute; DefaultApiVersion -> 2021-02-01; BuildImdsUri drops format=text; the 200-OK path deserializes the JSON body and reads location (then the existing ValidateRegion). Missing/empty/malformed body funnels to the existing FailedAutoDiscovery path. GetImdsUriApiVersionAsync and LocalImdsErrorResponse are reused unchanged.
• New DTO LocalImdsComputeResponse.cs with a single location property, registered in Platforms/net/MsalJsonSerializerContext.cs for source-gen.
• Tests (RegionDiscoveryProviderTests.cs): mock helper drops format=text / uses 2021-02-01; success bodies are now JSON; added missing-location and malformed-JSON scenarios. Lab mock helper + TestConstants.ImdsUrl updated to /compute.

Testing

• Microsoft.Identity.Client builds clean on netstandard2.0 and net8.0 (TreatWarningsAsErrors).
• Pass: RegionDiscoveryProviderTests (26), ClientCredentialWithRegionTests (6), RegionalTelemetryTests (6, unmodified), MtlsPopTests (73).
• No change to PublicAPI.Unshipped.txt.

Out of scope

Unified /compute snapshot cache for VM-only consumers (#6046); MI source detection.

[Copilot]

Pull request overview

This PR migrates MSAL.NET’s VM region auto-discovery from the legacy IMDS text endpoint (/metadata/instance/compute/location) to the newer IMDS JSON endpoint (/metadata/instance/compute), reading the location field while preserving the existing discovery/caching/retry behavior.

Changes:

• Update RegionManager to call /metadata/instance/compute?api-version=2021-02-01 and deserialize JSON to obtain location.
• Add a new LocalImdsComputeResponse DTO and register it for System.Text.Json source generation.
• Update unit/lab test helpers and constants to reflect the new endpoint and JSON response shape; add tests for missing location and malformed JSON.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/Microsoft.Identity.Test.Unit/CoreTests/RegionDiscoveryProviderTests.cs	Updates IMDS mocks to JSON /compute responses and adds new negative-path tests.
src/client/Microsoft.Identity.Lab.Api/PublicAPI.Shipped.txt	Updates shipped constant value for the IMDS region discovery URL to /compute.
src/client/Microsoft.Identity.Lab.Api/Internal/TestConstants.cs	Updates IMDS discovery URL constant to /compute.
src/client/Microsoft.Identity.Lab.Api/Http/MockHttpManagerExtensions.cs	Updates lab mock handler to return JSON with location.
src/client/Microsoft.Identity.Client/Platforms/net/MsalJsonSerializerContext.cs	Registers LocalImdsComputeResponse for source-generated JSON serialization.
src/client/Microsoft.Identity.Client/Instance/Region/RegionManager.cs	Switches to /compute and parses JSON location for region.
src/client/Microsoft.Identity.Client/Instance/Region/LocalImdsComputeResponse.cs	Introduces DTO representing IMDS /compute JSON payload (location).

Comments suppressed due to low confidence (1)

src/client/Microsoft.Identity.Client/Instance/Region/RegionManager.cs:246

• JsonHelper.DeserializeFromJson will throw on malformed JSON. While the outer catch (Exception e) will convert this into FailedAutoDiscovery, it will also change s_regionDiscoveryDetails/telemetry to an exception-specific string (and for a valid JSON body missing location, s_regionDiscoveryDetails stays null). To keep failure behavior/telemetry consistent with the existing “empty response” path, treat malformed JSON or missing location as an empty response and set s_regionDiscoveryDetails accordingly before falling back.

                            if (response.StatusCode == HttpStatusCode.OK && !response.Body.IsNullOrEmpty())
                            {
                                LocalImdsComputeResponse computeResponse = JsonHelper.DeserializeFromJson<LocalImdsComputeResponse>(response.Body);
                                region = computeResponse?.Location;

                                if (ValidateRegion(region, $"IMDS call to {imdsUri.AbsoluteUri}", logger))
                                {
                                    logger.Info(() => $"[Region discovery] Call to local IMDS succeeded. Region: {region}. {DateTime.UtcNow}");
                                    result = new RegionInfo(region, RegionAutodetectionSource.Imds, null);

[gladjohn]

Approved with comments

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.