fix: reject invalid multipart MIME header controls

[jbeckwith-oai]

Summary

• Reject invalid MIME-header control bytes in multipart field names, uploaded filenames, and content types before constructing multipart headers.
• Preserve valid Unicode, quotes, backslashes, horizontal tabs, supported public APIs, streaming, retry, content-length, and cancellation behavior.
• Add exhaustive encoder and CLI regression coverage, including nested JSON/YAML keys, every prohibited ASCII control byte, both filename interfaces, reader cleanup, unknown-length streams, and real newline-containing Unix filenames.

Validation

• GOCACHE=/Users/jbeckwith/.cache/openai-cli-go-build ./scripts/test -skip '^TestFilesCreateCLICancelClosesStalledFIFO$' — all repository packages passed; Windows/amd64 tests compiled successfully.
• GOCACHE=/Users/jbeckwith/.cache/openai-cli-go-build go test -race ./internal/apiform ./pkg/cmd -run '^(TestEncode|TestMarshal|TestMultipart|TestFilesCreateCLIRejectsNewlineFilenameBeforeRequest|TestFilesCreateCLIStreams|TestFilesCreateCLIDoesNotReplayUploads|TestImagesEditCLI|TestExactLength|TestOpenFileUpload|TestEmbedFiles)' -count=1
• GOCACHE=/Users/jbeckwith/.cache/openai-cli-go-build go vet ./...
• GOCACHE=/Users/jbeckwith/.cache/openai-cli-go-build ./scripts/lint
• GOCACHE=/Users/jbeckwith/.cache/openai-cli-go-build GOOS=linux GOARCH=amd64 go build ./...

TestFilesCreateCLICancelClosesStalledFIFO was excluded locally because its macOS EPIPE assertion also fails unchanged at the original base revision; Linux CI will run the complete unfiltered suite.

[Copilot]

Pull request overview

This PR hardens multipart form encoding in the CLI and internal apiform encoder by rejecting ASCII control bytes that can be used for MIME header injection (e.g., CR/LF) in multipart field names, uploaded filenames, and content types.

Changes:

• Add control-byte validation for multipart field names, filenames, and content types before constructing part headers.
• Add exhaustive security/regression tests for apiform multipart encoding, including nested decoded JSON/YAML keys and all prohibited ASCII control bytes.
• Add CLI-level regression tests ensuring invalid multipart header inputs are rejected before reading upload streams and before issuing HTTP requests.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
pkg/cmd/multipartheader_test.go	Adds CLI/multipart streaming tests to ensure invalid header controls are rejected early (no upload reads, owned readers closed, no requests sent).
internal/apiform/encoder.go	Introduces control-byte validation for multipart header-relevant values (field name, filename, content type) before part header construction.
internal/apiform/encoder_security_test.go	Adds comprehensive encoder security coverage for control-byte rejection and preservation of valid Unicode/escaping/tab behavior.

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[HAYDEN-OAI]

Independent deep security review at 0b79621 found no introduced P1/P2 issues.