fix(server): only redirect form posts to the Referer

[SulimanAbdulrazzaq]

Fixes #5445. Follow-up to #5428 / #5429.

Problem

ServerFunction::make_handler turns a successful response into 302 Found with Location: <Referer> whenever the request accepts text/html and carries a Referer. The comment says this is the Post/Redirect/Get step for a plain form post, but nothing checks the method, so it also fires on browser GET navigations:

• a link to a #[get] endpoint that returns a file (ByteStream) downloads the linking page instead of the file (Fullstack endpoint ByteStream response replaced by referer page data for a download link #5445);
• a callback-style #[get] endpoint reached by a redirect is bounced back to the page it came from (point 1 of #[get] server function error responses silently converted to 302 redirect when request has Referer header #5428; fix(server): don't redirect error responses from #[get] server functions #5429 fixed only the error-status part).

Fix

Apply the redirect only to POST requests. HTML forms can only submit GET or POST, so this keeps the form-post behavior and lets a GET navigation receive the handler's response.

Tests

Two unit tests in packages/fullstack-server/src/serverfn.rs send a browser-style request (Accept: text/html,..., Referer: http://localhost:8080/page) through make_handler:

• get_navigation_returns_the_response_instead_of_redirecting_to_referer: a GET gets 200 and the handler's body. Before this change it gets 302 to the Referer.
• plain_form_post_still_redirects_to_referer: a POST still gets 302 with Location set to the Referer.

cargo test -p dioxus-server --lib --bins --tests --examples, cargo clippy -p dioxus-server --tests -- -D warnings and rustfmt --check on the touched file pass.