feat: Update discriminator optimization when it has mixed fields

[mohankumarelec]

Summary

Extends the discriminator optimization from #292 to work with mixed schemas where only some alternatives have discriminators, rather than requiring all alternatives to have them.

Changes

• Relaxed discriminator coverage requirement: The optimization now applies when at least 2 alternatives (instead of all) share a discriminator property with const values

Test Coverage

• Added comprehensive tests with deeply nested self-referencing schemas (up to 1000 levels) to validate the optimization works correctly with complex discriminated unions.

Related

Extends #292

[mohankumarelec]

@aeschli , Please take a loot into this and let me know if it looks good, Thanks.

Feel free to edit the PR if required