[draft] Add null check on native create_index return and fix test callsites for new metricType param

[Copilot]

DiskANNService.CreateIndex was not validating the return value from the native create_index FFI call. A null pointer would silently propagate and crash later with an opaque error.

Changes

• DiskANNService.cs: Check create_index return for nint.Zero and throw GarnetException with a clear message. Since RecreateIndex delegates to CreateIndex, all callsites are covered.
• DiskANNServiceTests.cs: Update 3 create_index callsites to pass the new metricType parameter added in the v1.0.25 P/Invoke signature change.

var index = NativeDiskANNMethods.create_index(context, dimensions, reduceDims, quantType,
    (int)distanceMetric, buildExplorationFactor, numLinks, ...);
if (index == nint.Zero)
    throw new GarnetException("Failed to create DiskANN index, native create_index returned null");

[Copilot]

Pull request overview

Adds a defensive check around DiskANN native index creation to fail fast with a clear managed exception, and updates tests/callers to match the updated native create_index signature introduced by the diskann-garnet package bump.

Changes:

• Add nint.Zero validation for NativeDiskANNMethods.create_index and throw GarnetException on failure.
• Update DiskANNServiceTests callsites to pass the newly-required metricType parameter.
• Bump diskann-garnet package version from 1.0.23 to 1.0.25.

Reviewed changes

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

File	Description
libs/server/Resp/Vector/DiskANNService.cs	Adds null-pointer check after native create_index call; updates P/Invoke signature to include metricType.
test/Garnet.test/DiskANN/DiskANNServiceTests.cs	Updates direct create_index usages to include VectorDistanceMetricType argument.
Directory.Packages.props	Upgrades native DiskANN package dependency to 1.0.25.

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