GeezerVison

Fixes #2599.

Since #2465, the loader sets IncludeOrigin, so every schema records where it was declared. TypeDefinitionsEquivalent used reflect.DeepEqual, so identical schemas declared on different lines no longer matched.

This implements option 2 from my comment on the issue. It compares the schemas the same way as reflect.DeepEqual, but treats any two *openapi3.Origin values as equal. The code follows the standard library's deepValueEqual, including cycle detection for recursive schemas, and its doc comment lists where the two differ. Fields are skipped by type, so this also covers Origin on SchemaRef, Discriminator and other kin-openapi types, and on any fields added later. It adds no new dependency. If you'd rather use go-cmp, I'm happy to switch, and the tests stay the same.

Tests:

• TestTypeDefinitionsEquivalentIgnoresOrigin covers inline, nested and recursive schemas, which should be equal. It also covers two cases that must stay different: the same relative $ref resolving to different files, and a 3.1 $ref with a sibling keyword. Comparing JSON would fail both of these.
• TestTypeDefinitionsEquivalentCycles compares two separately built self-referencing schemas. A loaded spec shares one *Schema per $ref target, so the loaded cases never reach the cycle detection. Without the detection, this test overflows the stack.
• TestIdenticalXGoTypeNameSchemasWithOrigin runs the issue's spec through Generate.

make generate produces no diff with Go 1.27.1, which matches CI's stable. With Go 1.25 or 1.26, only the embedded spec blobs change, with or without this change. make test and make lint pass.

I used Claude to help write this change. I've reviewed and tested it myself.