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

[typescript-bot]

With 6.0 out as the final release vehicle for this codebase, we're closing all PRs that don't fit the merge criteria for post-6.0 patches. If you think this was a mistake and this PR fits the post-6.0 patch criteria, please post to the 6.0 iteration issue with details (specifically, which PR and which patch criteria it satisfies).

Next steps for PRs:

• For crash bugfixes or language service improvements, PRs are currently accepted at the typescript-go repo
• Changes to type system behavior should wait until after 7.0, at which point mainline TypeScript development will resume in this repository with the Go codebase
• Library file updates (lib.d.ts etc) continue to live in this repo or the DOM Generator repo as appropriate