Handle native FormData in edge runtimes

[alloutflo]

Summary

• identify npm form-data bodies by their stream and getHeaders() capabilities instead of constructor identity
• keep npm form-data uploads on the Node transport while allowing WHATWG FormData to use native fetch
• add an edge-runtime regression that reproduces the reported body.getHeaders is not a function failure with a shimmed FormData constructor

Fixes #696.

Testing

• full Jasmine suite: 85 specs passed
• existing npm form-data multipart request test passed
• new WHATWG FormData runtime-shim request test passed
• npm run lint
• npm run build

AI assistance disclosure

OpenAI Codex was used to analyze, implement, and test this change. The diff and checks listed above were verified in the contribution workspace.

[alloutflo]

@ksvirkou-hubspot Maintainer handoff: this PR is ready for review, mergeable, up to date with master, and has no review comments or open threads. The repository’s lint and test workflows are push-only, so GitHub does not create an upstream PR check here. The full local Jasmine suite passes (85 specs), including both the existing npm form-data path and the new WHATWG FormData edge-runtime regression; lint and build also pass. Could you review the transport capability check when convenient?