fix: preserve context types in check-event deduplication

[roncohen]

Summary

Apply the relevant context-identity improvement from JavaScript PR #712 without adding a remote-evaluation API:

• Replace flattened query-string check-event deduplication keys with canonical JSON.
• Preserve JSON types, empty arrays, nested object structure, and array order.
• Recursively sort object keys, including objects inside arrays, so equivalent contexts share a deduplication key.
• Add integration/unit regressions for type collisions, dotted keys versus nested paths, array indices versus object keys, and stable nested ordering.

This PR is independent of the array-targeting/diagnostics port and does not require backend transport changes. It changes only internal check-event identity, not the event payload or legacy FlattenJson behavior.

Validation

• dotnet build dotnet-sdk.sln -c Release: zero warnings/errors; library builds for .NET 10 and .NET Standard 2.0.
• dotnet test dotnet-sdk.sln -c Release --no-build: 270 passed (130 SDK + 5 example tests on each of .NET 8 and .NET 10).
• git diff --check passed.
• Verified conflict-free merge with the separate array-targeting/diagnostics branch.

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