feat!: response headers, type-safe styles, multi-content-type

[maxholman]

Summary

• Header construct: Full HeaderObject implementation with synth(), referenceObject(), components/headers registration, and export from package index
• HTTP/2 lowercase enforcement: Header IDs, Parameter<_, 'header'> names, Response.headers keys, and ValidParameter all reject uppercase at compile time via Lowercase<string> constraints
• Type-safe parameter styles: StyleForIn<TIn> constrains style values per OAS 3.1 location (e.g. query gets form | spaceDelimited | pipeDelimited | deepObject, header gets simple). Added missing explode field
• Multi-content-type support: Response and RequestBody accept content as single value or array, enabling endpoints with multiple representations (e.g. JSON + binary)
• Widened MIME types: MediaType.contentType accepts any string while preserving autocomplete for common types (application/json, application/octet-stream, text/plain, etc.)

Breaking changes

• Response.headers type changed from Header[] to Record<Lowercase<string>, Header>
• Parameter style field now constrained per in location (previously only accepted 'simple')
• ValidParameter header variant now requires Lowercase<string> names

Test plan

• tsc --noEmit passes
• All vitest tests pass with updated snapshots
• OpenAPI validation passes via @apidevtools/swagger-parser
• Type tests verify lowercase header enforcement (positive + @ts-expect-error negative)
• Example fixture exercises: reusable headers on responses, multi-content-type response (JSON + octet-stream), binary upload request body