[release/10.0] Always generate HasDiscriminator with type argument and property name in snapshot

[Copilot]

Fixes #37143

Description
When a discriminator property has a non-string type (e.g., int) and uses the default property name "Discriminator", the model snapshot generator would omit the type argument. This caused the discriminator type to default to string when reading the snapshot back, resulting in spurious migrations being generated on every scaffold.

// Before (loses type info, defaults to string on read-back)
b.HasDiscriminator().HasValue(0);

// After (preserves int type)
b.HasDiscriminator<int>("Discriminator").HasValue(0);

Customer impact
Customers using non-string discriminator types with the default "Discriminator" property name would see new migrations generated each time they ran dotnet ef migrations add, even when no actual model changes were made.
A workaround is to fix the snapshot manually, but this would need to be performed after every new migration. Another workaround is to use a custom discriminator property name.

How found
Customer reported on 10.0.0.

Regression
Yes, from EF Core 9.0. Introduced in #35744

Testing
Test added

Risk
Low. The change is minimal, only affects snapshot generation output, which is a design-time action

Original prompt

Fix #37143 by always generating a HasDiscriminator call with generic type argument and property name in the snapshot.

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[AndriySvyryd]

@copilot Update progress by rewriting the PR summary using this template:

Fixes #{number}

**Description**
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**Customer impact**
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**Regression**
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[Copilot]

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Updated the PR description using the requested template.

[AndriySvyryd]

cc @artl93

[artl93]

Customer Reported regression.