Support form-encoded requests

[t1]

Add support for form-encoded requests.

[t1]

Agent: starting work

Type: Feature
Assessment: Add support for operations that accept application/x-www-form-urlencoded request bodies. Currently only application/json is supported.

Design decisions:

• Add a Content-Type selector (like the existing Accept selector) when an operation accepts multiple request body types
• For form-encoded bodies, render individual input fields (one per schema property) instead of a JSON textarea
• Serialize form data to URL-encoded format on submit
• Reuse existing schema traversal logic to generate field inputs

Acceptance criteria:

• Operations with form-encoded request bodies render individual input fields
• Form data is correctly serialized as application/x-www-form-urlencoded
• Content-Type selector appears when both JSON and form-encoded are available
• Demo app includes an operation that exercises form-encoded requests

Files:

• core/src/main/java/com/github/t1/openapi/ui/generator/OperationFragmentGenerator.java (request body rendering)
• core/src/main/resources/openapi-ui.js (form serialization)
• core/src/test/java/com/github/t1/openapi/ui/generator/BrowserTest.java (browser tests)
• Demo app spec (add form-encoded endpoint)

[t1]

Agent: done

Commit: 0318093
Summary: Added support for application/x-www-form-urlencoded request bodies. Operations with form-encoded content now render individual input fields (text for strings, checkboxes for booleans) instead of a JSON textarea. JavaScript serializes the form data as URL-encoded on submit. Added demo endpoint /pets/search that accepts form parameters.
Tests: Added shouldRenderFormFieldsInsteadOfTextarea test in BrowserTest$GivenAppWithFormEncodedRequestBody using existing form-encoded.yaml spec. Test verifies form fields render and textarea is not present. Added screenshot capture for visual verification.
CI: passed