multipart/form-data bodies with file parts #30

Why

Uploads are the most common thing a plain .http file cannot express today. REST Client and JetBrains both accept a multipart body written out with the boundary and < ./file inside a part; apic sends it as a literal string, so the file reference goes over the wire as text. The cookbook's "uploads" recipe is limited to a whole-body < file for the same reason.

What

Support the editor syntax as it stands:

POST {{baseUrl}}/upload
Content-Type: multipart/form-data; boundary=WebAppBoundary

--WebAppBoundary
Content-Disposition: form-data; name="title"

Quarterly report
--WebAppBoundary
Content-Disposition: form-data; name="file"; filename="report.pdf"
Content-Type: application/pdf

< ./report.pdf
--WebAppBoundary--

Acceptance criteria:

• When Content-Type is multipart/form-data, the body is parsed into parts; a < path or <@ path line inside a part is replaced by the file's bytes (templated for <@). Paths are relative to the .http file and confined to the project root exactly like bodyFilePath (internal/runner/runner.go:817-845).
• The body is assembled binary-safe (a []byte, not a string round-tripped through templates). Content-Length is set.
• {{variables}} in text parts resolve as usual.
• apic validate reports a missing part file as an error, like it does for a whole-body file.
• apic curl maps parts onto -F name=value / -F file=@path.
• --json shows the request body as "<multipart: 2 parts, 1 file>" rather than the bytes; --redact behaves as for other bodies.
• The OpenAPI importer generates a multipart skeleton for multipart/form-data request bodies instead of skipping them (internal/openapi/import.go:498-500).

Where

• internal/httpfile/parse.go (body capture at 189-203), ast.go (Request.Body)
• internal/runner/runner.go (body construction 289-310, send 520-523)
• internal/curlexport/curl.go
• internal/openapi/import.go

Docs

docs/format.md (Structure table, new "Multipart" section), docs/cookbook.md (replace "Send a file as the body" with an upload recipe), docs/comparison.md table.

Tests

Parser case in internal/httpfile/testdata/sample.http; runner test with httptest reading the request via multipart.NewReader and checking part names, filenames and bytes; validate test for a missing part file; curl export test.