fix(cli): parse OpenAPI oneOf with discriminator as discriminated union

[thesandlord]

Description

Refs: Related to fern-platform PR #6485

Fixes OpenAPI oneOf schemas with discriminator being incorrectly parsed as undiscriminated unions when the discriminant property is defined in each variant schema (which is the standard OpenAPI pattern).

Changes Made

• Modified OneOfSchemaConverter.ts to always parse oneOf with discriminator as a discriminated union, regardless of whether the discriminant property exists in variant schemas
• Added filterDiscriminantFromSchema() method to remove redeclared discriminant properties from variant schemas, keeping the union properly discriminated without duplicate properties
• When a variant schema contains the discriminant property, it's now filtered out and the schema is converted as an inlined type with the original type ID
• Updated test snapshots to reflect the new discriminated union output format

Context

The previous logic (from PR #6738) checked if variant schemas contained the discriminant property, and if they did, it would convert to undiscriminatedUnion(). This was a workaround to avoid duplicate discriminant properties, but it lost the discriminated union semantics.

The new approach:

1. Always converts oneOf with discriminator as a discriminated union
2. Filters out the discriminant property from variant schemas when it's redeclared
3. Preserves the original type IDs so SDK type names remain unchanged

Testing

• Existing unit tests pass (v3-importer-commons)
• Snapshot tests updated for v3-sdks.test.ts
• Snapshot tests updated for openapi-from-flag.test.ts
• Lint checks passing

Human Review Checklist

• Verify the filterDiscriminantFromSchema logic correctly handles the required array
• Check if schemas with allOf compositions are handled correctly when filtering
• Verify the unionVariantsContainLiteral function can be removed (now unused)
• Confirm generated SDKs work correctly with the filtered variant schemas

Link to Devin run: https://app.devin.ai/sessions/f05a2106646045aa9c97af3bf986fbec
Requested by: Sandeep Dinesh (@thesandlord)

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