feat(cli): add support for multiple override files in OpenAPI spec configuration

[jsklan]

Description

Refs: User request from [email protected] (@jsklan)

Adds support for specifying multiple override files in OpenAPI spec configuration. The overrides field now accepts either a single path (for backwards compatibility) or an array of paths that are applied sequentially in order.

Additionally, adds support for generator-level overrides, allowing each generator invocation to specify its own overrides field that applies additional override files after the top-level spec overrides.

Link to Devin run: https://app.devin.ai/sessions/8d80e6afe9b840e6aee1406cfd78569c

Changes Made

Multiple Override Files (Top-Level)

• Added new OverridesSchema union type in the Fern definition that accepts string | list<string>
• Updated 6 schema definitions (APIDefinitionWithOverridesSchema, ProtobufDefinitionSchema, OpenAPISpecSchema, OpenRPCSpecSchema, AsyncAPISpecSchema) to use the new type
• Regenerated TypeScript types from the schema
• Updated GeneratorsConfiguration.ts to store overrides as string | string[] | undefined
• Updated all spec types in Spec.ts (OpenAPISpec, OpenRPCSpec, ProtobufSpec) to support array of override paths
• Modified loadAPIWorkspace.ts to convert relative paths to absolute paths for both single and array formats, with file existence validation for all paths
• Modified loadOpenAPI.ts to iterate through multiple override files sequentially using the existing mergeWithOverrides function
• Modified loadAsyncAPI.ts to handle array overrides with sequential application
• Modified parseOpenAPI.ts to accept array type (uses first override path for ref resolution)
• Modified OSSWorkspace.ts to handle array overrides in path conversion and file path collection

Generator-Level Overrides

• Added overrides field to GeneratorInvocationSchema in group.yml
• Updated GeneratorInvocation interface in GeneratorsConfiguration.ts
• Updated convertGeneratorsConfiguration.ts to parse the new field
• Added mergeGeneratorOverridesWithSpecs function in runLocalGenerationForWorkspace.ts to merge generator-level overrides with spec-level overrides
• Added changelog entry for version 3.38.0 with both features

Bug Fixes

• Added missing overrides: undefined property to getGeneratorInvocation.ts in seed package to fix TypeScript compilation error

Testing

• Unit tests added/updated
• Manual lint check completed
• TypeScript compilation passes

Human Review Checklist

• Sequential override logic: Verify the for-loops in loadOpenAPI.ts, loadAsyncAPI.ts, and loadOpenRpc.ts correctly apply overrides in order
• Ref resolution limitation: In parseOpenAPI.ts, only the first override path is used for the OpenAPIRefResolver. Verify this is acceptable behavior if refs are defined in later override files.
• Generator override merging: In runLocalGenerationForWorkspace.ts, the mergeGeneratorOverridesWithSpecs function returns undefined if specsOverride is null but generator overrides exist. Verify this is the intended behavior (generator overrides only work when apiOverride.specs is also defined).
• Spec type limitation: Generator-level overrides are only applied to OpenAPI specs (via isOpenApiSpecSchema check). Verify this is acceptable or if other spec types should also support generator overrides.
• Consider adding tests: No seed test fixture was created for these features

CI Notes

Some CI checks (python-sdk, php-model) are failing due to a pre-existing @fern-api/go-formatter compilation issue unrelated to these changes. All other seed tests pass.

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