Support multi-signer multipart send_document requests

[holtbp]

Summary

send_document's multipart path (used whenever files: is passed, i.e. uploading a fresh PDF rather than referencing an existing template) only ever unwrapped the single-signer/single-file case — a second signer in signers: raised NotImplementedError. This adds support for sending 2+ signers (e.g. a sequential-order envelope) alongside an uploaded file.

Confirmed against BoldSign's own docs that the correct multipart encoding for a multi-valued field is one part per element, all sharing the same field name (e.g. two Signers= parts) — not a single field whose value is a JSON array, and not the bracket-suffixed Signers[]= form Faraday's multipart middleware emits by default for Array values (which BoldSign rejects, same as the existing Files[] gotcha this gem already works around).

What changed

• Client#connection now passes flat_encode: true to Faraday::Multipart::Middleware, which is what makes an Array value emit as repeated same-named parts instead of key[]=.
• Resources::Document#encode_multipart_value now maps every element of an Array value to its own JSON-encoded part, instead of special-casing (and raising past) the single-element case. Functionally identical output for today's single-signer callers — flat_encode + a one-element Array produces the exact same single part as the old unwrap-and-raise-if-more logic (covered by the existing "sends a multipart request when files: is provided" spec, unchanged and still passing).
• Multi-file uploads are a separate concern and still raise NotImplementedError — out of scope here.

Testing

• Existing suite passes unchanged (100% line/branch coverage maintained).
• Replaced the old "raises for multi-element arrays" assertion with a new spec sending 2 signers + signerOrder + enableSigningOrder: true alongside a file upload, asserting exactly 2 signers parts with no [] suffix and no wrapping JSON array.
• Kept a narrower multi-file-specific raise test (unaffected by this change).

Context

We're building a sequential 2-signer BoldSign envelope (an uploaded PDF, not a template) at EscrowSafe and hit this exact NotImplementedError. Happy to adjust the approach if you'd prefer a different API shape (e.g. an explicit signers: array vs. relying on flat_encode globally for all multipart Array values — I don't see another multipart caller in the gem today where that would change behavior, but flagging in case future callers expect bracket-style array semantics for something else).