fix: simplify isBinaryPayload to use encode="bytes" as binary signal

[Copilot]

fixes #3913

In TCGC, the encoding for a bytes response body is determined as follows:

For */* and application/octet-stream content types, TCGC natively sets the encode to "bytes".
For all other content types (e.g., application/json, application/xml), a bytes response body defaults to "base64" encoding — unless the user explicitly annotates the body with @encode("bytes"), which overrides the encode to "bytes".
Given this behavior, the isBinaryPayload logic can be simplified: if encode === "bytes" and the content-type is not text/plain, return true directly; otherwise, fall through to the original logic. This approach:

Naturally handles the */* case — since TCGC already sets encode to "bytes" for wildcard and octet-stream content types, no special-casing for */* is needed anymore.
Correctly handles explicit @encode("bytes") annotations on non-binary content types (e.g., application/json + @encode("bytes") → treated as binary).
Preserves the text/plain exception — even with @encode("bytes"), text/plain should not be treated as a binary stream.
Keeps backward compatibility with m4 → TCGC migration, since the encode field reliably reflects the wire-level intent.
The dedicated */* wildcard detection code path can therefore be removed in favor of this unified encode-based check.