openapi: split content.go by construct #1059

Problem

compilers/openapi/internal/operation/content.go (1,142 lines) holds media types, multipart and sequential encoding, headers, examples and request bodies, and the package comment.

Direction

• encoding.go: part encodings, multipart body parts, sequential (itemEncoding/prefixEncoding) encoding, and the body-model helpers only they use.
• headers.go: headers and their serialization.
• examples.go: example lists and spellings.
• body.go: the request body.
• content.go keeps media types and content entries. The package comment moves to doc.go.
• Tests follow their names, including the three TestContent_* tests openapi: split operations.go by construct #1057 left in operations_test.go.
• internal/resolve/resolve.go cites bodySchemaPointer (content.go); it becomes operation/encoding.go.

Blocked by

#998, #1000 and #1046 edit content.go or its tests; #1057 rewrites operations_test.go.

Done when

• Every top-level declaration of the package, with its doc comment and any comment between declarations, is byte-identical before and after the move (compared with a throwaway go/ast script), and no golden changes.
• Comments that name a file name the file the code now lives in, changed in a commit of their own.
• go test ./compilers/openapi/... passes without -update.

Priority: P4, rule 1 (housekeeping: moves code between files and changes no lowering, validation, IR or emit-contract behaviour). wc -l compilers/openapi/internal/operation/content.go → 1142 (morphic @a1f15b29).