[followdarko]

Closes #3731.

The problem

fetch sends the request body inside the JSON arguments of plugin:http|fetch as Array.from(new Uint8Array(buffer)): one number per byte, built and stringified on the webview's main thread, then parsed back by serde_json. The window stalls for as long as that takes, which grows with the body.

The change

• The guest code sends a non-empty body as the raw IPC payload, invoke(cmd, new Uint8Array(buffer), { headers }), and the rest of the request options as percent-encoded JSON in a client-config IPC header. plugin:fs|write_file passes file contents and options the same way.
• fetch takes tauri::ipc::Request and reads the options from that header and the body from InvokeBody::Raw, or from a JSON number array, which is how a raw payload arrives on Android and under the postMessage fallback.
• Without the header it reads { clientConfig } from the JSON arguments as before. The guest code still uses that form for a request without a body, and it keeps older guest code working against this crate.

Android and the isolation pattern get no speed-up, since a raw payload travels as a number array there, but they take the same code path.

Verification

A small app on macOS 15.7 (arm64, WKWebView, release build) sends the same requests through the previous api-iife.js and through this one, both against this crate, to a local server that answers with the body's length and hash. "Stall" is the longest gap between event-loop turns while the request is in flight. Median of 10 requests per row:

body	stall, number array	stall, raw payload	whole request, number array	whole request, raw payload
16 KiB	3 ms	1 ms	8 ms	5 ms
64 KiB	10 ms	1 ms	19 ms	6 ms
900 KiB	110 ms	2 ms	155 ms	11 ms
4 MiB	881 ms	8 ms	1010 ms	26 ms
16 MiB	4131 ms	48 ms	4777 ms	131 ms

performance.now() in WKWebView has 1 ms resolution, so the small rows are at its floor.

The same app checks that binary and text bodies arrive byte for byte, that a byte-string header value keeps its bytes and the content type is sent, that a POST without a body and a GET still go through, and that the previous guest code still works against this crate.

Unit tests cover reading the options and the body: a raw payload, a number array, an out-of-range number, the JSON arguments with and without data, and a raw payload without the header. The existing api-e2e http specs (JSON, binary, multipart and byte-string header bodies) all go through the new path.

cargo test -p tauri-plugin-http, cargo clippy -p tauri-plugin-http --all-targets -- -D warnings, cargo fmt --check, pnpm build, eslint and prettier --check pass on v2.