fix(openapi): widen global header type to string when schemas are incompatible

[tjb9dc]

Description

Refs https://buildwithfern.slack.com/archives/C09NRQZ4A2X/p1765250090217619

Fixes a bug where an OpenAPI spec with the same header name (e.g., Accept) across multiple endpoints but with different schemas causes validation errors during example generation.

Root cause: When detecting global headers, the importer uses the first endpoint's schema for all endpoints. If subsequent endpoints have different schemas for the same header (e.g., one with default: 'text/event-stream' and another with const: 'application/json'), the generated examples don't match the global header's type, causing validation errors like:

Expected example to be "application/json". Example is: "text/event-stream"

Fix: When incompatible schemas are detected for the same header name across endpoints, widen the header type to string instead of using a literal type.

Changes Made

• Added markIncompatible() and isIncompatible() methods to HeaderWithCount class to track schema incompatibility
• Added getTypeFromHttpHeaderSchema() helper to extract type from Fern header schema for comparison
• Added widenToString() helper to convert header schema to plain string type
• Added logic to detect when headers with the same name have different schemas across endpoints
• When incompatible schemas are detected, the global header type is widened to string
• Added test fixture openapi-accept-header to reproduce the issue
• Added JSON schema snapshots for the test fixture
• Added Swift SDK endpoint path snapshot for the test fixture

Testing

• Manual testing: Verified the test fixture passes fern ir without validation errors
• Lint checks pass
• JSON schema snapshot tests added
• Swift SDK snapshot tests added

Human Review Checklist

• Verify the schema comparison logic in getTypeFromHttpHeaderSchema is sufficient (currently only compares the type field from the converted Fern schema)
• Consider if there are edge cases where widening to string loses important type information
• Confirm the test fixture adequately covers the reported bug scenario

---

Link to Devin run: https://app.devin.ai/sessions/dab9b93cc55a4f79ae9e7453b2e024ad
Requested by: [email protected] (@tjb9dc)

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring