fix(openapi): propagate webhook responses from OpenAPI to IR

[sbawabe]

Fixes FER-8007 (pt3)

Description

Refs: Requested by Sarah Bawabe (@sbawabe)

Webhook responses defined in OpenAPI specs (e.g., 204, 400, 500 status codes) were not being captured in the IR. This PR adds support for extracting and storing webhook responses in the IR.

Link to Devin run: https://app.devin.ai/sessions/f101e52b58ab44b49d44dc2bc1fd5c81

Changes Made

• Added responses optional field to IR Webhook type definition (webhooks.yml)
• Reuses existing http.HttpResponse type instead of creating a new type (per PR feedback)
• Updated WebhookConverter to process operation.responses from OpenAPI specs
• Added support for wildcard status codes (e.g., 4XX → 400, 5XX → 500) via isWildcardStatusCode field
• Added responses: undefined to convertWebhookGroup.ts for Fern-defined webhooks (no response source available)
• Regenerated IR SDK types
• Updated test snapshots across multiple packages (register, v3-importer-tests, ir-generator-tests, ete-tests)
• Added IR migration in v62-to-v61/migrateFromV62ToV61.ts to strip responses field and headers from errors for backward compatibility
• Added changelog entry in versions.yml for version 3.9.0

Updates Since Last Revision

• Merged main branch after IR version 63 was deleted
• Moved migration logic from v63-to-v62/migrateFromV63ToV62.ts to v62-to-v61/migrateFromV62ToV61.ts
• Deleted the v63-to-v62 migration file
• Migration now also strips headers field from errors for v61 compatibility
• Bumped changelog version from 3.8.1 to 3.9.0 (changelog validator requires minor version bump for feat type changes, not patch)
• Resolved merge conflict with main after 3.8.0 was released

Testing

• Unit tests updated (snapshots updated for webhook fixtures)
• Lint checks pass
• Local compilation verified (ir-generator, ir-migrations, and openapi-to-ir compile successfully)

Human Review Checklist

• Verify the IR schema change is backward compatible (new field is optional, migration strips it for v61)
• Confirm body: undefined is acceptable for webhook responses (webhooks don't have response bodies like endpoints)
• Review wildcard status code conversion (4XX → 400, 5XX → 500 with isWildcardStatusCode: true)
• Note: Response description from OpenAPI is not captured since HttpResponse doesn't have a docs field - confirm this is acceptable
• Confirm if FDR propagation is needed - currently responses are captured in IR but not yet surfaced in FDR output

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