openapi-codegen: support components.headers

[magdzikk]

openapi-codegen decodes components.parameters but not components.headers. The section was silently dropped at decode time, and OpenapiHeaderRef.resolved only ever consulted components.parameters (additionally requiring in: header), so a response header written as $ref: '#/components/headers/Foo' passed ref validation and then failed at generation with IllegalStateException: Response component #/components/headers/Foo is referenced but not found.

This adds headers to OpenapiComponent, re-keyed by full ref exactly as parameters already is; makes OpenapiHeaderRef.resolved consult components.headers first and fall back to components.parameters unchanged; and adds the missing headers case to OpenApiMerger.

Documents that do not use components.headers are unaffected.

Tests: openapiCodegenCore2_12/test 105 passing, openapiCodegenCore3/test 115 passing, scalafmtCheckAll clean.

🤖 Generated with Claude Code