Fix enum discriminator serialization inconsistency in Cosmos provider

[Copilot]

The Cosmos provider was incorrectly serializing enum discriminators as integers during entity updates, even when configured with a string converter. This caused entities to become unqueryable after updates.

Problem

When using enum discriminators with string conversion in the Cosmos provider:

1. First save: Discriminator correctly saved as string (e.g., "ThingyTwo")
2. Subsequent updates: Discriminator incorrectly saved as integer (e.g., 1)

This inconsistency broke queries that expected string discriminator values, making updated entities unloadable.

// Configuration
modelBuilder.Entity<Thingy>(b =>
{
    b.Property(e => e.Type).HasConversion<string>();
    b.HasDiscriminator(e => e.Type)
        .HasValue<DerivedThingy2>(ThingyType.ThingyTwo);
});

// After first save: { "Type": "ThingyTwo" } ✓
// After update: { "Type": 1 } ✗

Root Cause

The issue was in DocumentSource.ConvertPropertyValue() which used entry.GetCurrentProviderValue(property) for all properties. During updates, change tracking could corrupt discriminator property values, causing the method to return the raw enum integer instead of the converted string value.

Solution

Enhanced ConvertPropertyValue() to handle discriminator properties specially:

• Detect discriminator properties using entry.EntityType.FindDiscriminatorProperty()
• For discriminator properties, use the authoritative value from entry.EntityType.GetDiscriminatorValue()
• Apply the property's type mapping converter to ensure correct serialization
• Preserve existing behavior for all non-discriminator properties

This ensures discriminator values remain consistent regardless of change tracking state.

Fixes #34852.

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