fix(spec): handle requestBody with no content without crashing

[yogeshwaran-c]

Description

When a requestBody object has no content property (only description and required), the requiredParameters selector in src/core/plugins/spec/selectors.js crashes with:

TypeError: Cannot read properties of undefined (reading 'entrySeq')

This happens because requestBody.getIn(["content"]) returns undefined when there is no content key, and .entrySeq() is then called on undefined.

The fix adds a null check before iterating over content entries, returning the partial result early when content is absent.

Motivation and Context

Fixes #10767

While the OpenAPI spec expects requestBody to have a content field, real-world specs sometimes omit it (e.g., when using code generators or incomplete specs). Swagger UI should handle this gracefully instead of crashing.

How Has This Been Tested?

• Verified the fix prevents the TypeError when requestBody has no content
• The early return preserves any required field that was already extracted from the requestBody
• Existing behavior is unchanged when content is present

Screenshots (if appropriate):

N/A

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.

[lukaszzazulak]

@yogeshwaran-c Although the content field is required (https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.0.md#request-body-object), the fix is valid since the app should not crash despite errors in the document; please just include a unit test for this case to prevent future regressions.

[yogeshwaran-c]

@lukaszzazulak Thanks for the review and for confirming the fix is valid. I've added a unit test for getOAS3RequiredRequestBodyContentType in test/unit/core/plugins/spec/selectors.js that covers the no-content case (along with a couple of related cases for context) — pushed in 9aa5940.