test: cover sending a Buffer Actor input as raw bytes - #1078

Adds a regression test for #277: `start()` with a `Buffer` input and
`contentType: 'application/json'` sends the raw bytes, and the server
gets the decoded JSON back.

The bug itself is already gone on `v3`. `serializeBody()` from #1058
passes binary and string bodies through before it reaches
`JSON.stringify`, and axios no longer re-serializes strings since #1051
set `transformRequest: []`. With the Buffer branch disabled, the new
test fails with the `{ type: 'Buffer', data: [...] }` payload from the
issue.

Closes #277

*✍️ Drafted by Claude Code*

#1099)

`ActorInput` was an alias for `object`, so a `Date`, `Map`, `Set` or
function compiled and then went out as a wrong body or threw. It's now
`Dictionary | Dictionary[] | Uint8Array`, which is the type suggested in
#1054 plus `Uint8Array`. The extra member keeps raw-bytes input
compiling (a `Buffer` sent with a `contentType`, see #277 and #1078).

This is a breaking change for input declared as an `interface`.
Interfaces get no implicit index signature, so they don't match
`Dictionary`. The v3 migration guide shows the fix, either a `type`
alias or a spread (`{ ...input }`).

A type test pins the accepted and rejected shapes.

This doesn't touch `contentType`, the other half of #1054. The platform
still stores the `INPUT` record under the content type it receives.

Closes: #1054

*✍️ Drafted by Claude Code*