Fix #4493: NullReferenceException combining optimistic concurrency and partitioning

[jeremydmiller]

Summary

Closes #4493.

UpsertFunction's partition-column scan called arg.Column.Equals(colName, ...) on each UpsertArgument when building the partition WHERE-clause fragment. CurrentVersionArgument deliberately sets Column to null (it's a sentinel that doesn't map to a stored column), so when optimistic concurrency was enabled on a document that also participated in AllDocumentsAreMultiTenantedWithPartitioning(...), DocumentSchema construction blew up with NullReferenceException during ApplyAllConfiguredChangesToDatabaseAsync — exactly the symptom in the reporter's repro.

The fix

-                var arg = Arguments.FirstOrDefault(a =>
-                    a.Column.Equals(colName, StringComparison.OrdinalIgnoreCase));
+                var arg = Arguments.FirstOrDefault(a =>
+                    string.Equals(a.Column, colName, StringComparison.OrdinalIgnoreCase));

string.Equals(null, "foo", ...) is null-safe and returns false, so a CurrentVersionArgument is naturally skipped — the only path that was crashing.

Test plan

• New regression test CoreTests/Partitioning/Bug_4493_optimistic_partitioning.cs reproduces the exact configuration from the reporter's repro (.UseOptimisticConcurrency(true) + AllDocumentsAreMultiTenantedWithPartitioning with two list partitions) and round-trips a document through one of the partitions.
• Fails with NullReferenceException at UpsertFunction.cs:127 without the fix (verified locally).
• Passes with the fix on net8.0 / net9.0 / net10.0.
• Adjacent partitioning tests (CoreTests.Partitioning.*) all 27 still pass.

Master companion PR will follow.

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