[rc2] Fix Cosmos enum discriminator handling

[Copilot]

Fixes #36329
Fixes #34852

Description
When using an enum discriminator in the Cosmos DB provider, querying a DbSet for derived types would throw an InvalidCastException. This occurred because the discriminator comparison logic was using GetProviderValueComparer() which is meant to compare the provider representations of values, in this case string. The fix is to use GetValueComparer() as that's the comparer for client-side values.

Customer impact
Customers using enum discriminators in Cosmos DB cannot query derived types via DbSet. The workaround is to query through the base entity type instead, however if any property is modified the enum discriminator is saved as a number, resulting in data corruption.

How found
2 customer reports.

Regression
Yes, the query issue is a regression from EF 8, the SaveChanges issue is a regression from EF 7.

Testing
Test added.

Risk
Low. For properties without value converters (the vast majority) GetValueComparer and GetProviderValueComparer return the same instance, so this change only affects the broken case.

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[AndriySvyryd]

@copilot Rewrite the PR summary using this template:

Fixes #{number}

**Description**
{Issue description}

**Customer impact**
{How does the reported issue affects customer's app? Are there workarounds?}

**How found**
{Was it customer reported or found during verification? How many customers are affected?}

**Regression**
{Is it a regression from a released version? Which one?}

**Testing**
{How the changes were tested}

**Risk**
{Low/Medium/High, argument why}

[AndriySvyryd]

@copilot This isn't the right fix. You should just change GetProviderValueComparer() to GetValueComparer()) and leave the rest of the code as it was

[Copilot]

You're absolutely right! I've simplified the fix to just change GetProviderValueComparer() to GetValueComparer() as suggested. This is much cleaner and achieves the same result by comparing CLR values directly instead of trying to convert and compare provider values. Fixed in commit 2f62f21.

[Copilot]

@copilot Rewrite the PR summary using this template:

...

Updated the PR description using the requested template:

Fixes #36329

Description
When using an enum discriminator with a value converter (such as EnumToStringConverter) in the Cosmos DB provider, querying a DbSet for derived types would throw an InvalidCastException: "Unable to cast object of type 'MyEnum' to type 'System.String'". This occurred because the discriminator comparison logic was using GetProviderValueComparer() which expected both values to be of the same provider type, but one was a CLR enum and the other was the converted provider value.

Customer impact
Customers using enum discriminators with value converters in Cosmos DB cannot query derived types via DbSet. The workaround is to avoid using enum discriminators with value converters or query through the base entity type instead.

How found
Customer reported via GitHub issue #36329.

Regression
No - this appears to be a long-standing issue with enum discriminators and value converters in Cosmos DB.

Testing
Existing test Enum_discriminator_with_value_converter_on_derived_dbset validates the fix. The change uses CLR value comparison instead of provider value comparison, which handles both converted and non-converted discriminator scenarios correctly.

Risk
Low - minimal one-line change replacing GetProviderValueComparer() with GetValueComparer() to compare CLR values directly, which is the appropriate comparison for this scenario.

[artl93]

RC2