Improve error messages for discriminated union incompatible members

[alpha2420]

Fixes #62737

When assigning objects with wider discriminator unions to discriminated union types, TypeScript now identifies the actual incompatible types from the source union, rather than showing a misleading error about a compatible type.

Before: Type '"foo"' is not assignable to type '"bar"'

After: The type(s) '"baz"' are not present in the discriminator 'discriminator' of the target.

This change:

• Adds detection for discriminated union assignment errors
• Reports which specific types from the source are missing in the target
• Includes the discriminator property name in the error message
• Adds comprehensive test cases

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