Rollup merge of #150881 - fix-wasi-fs-copy, r=alexcrichton

Fix std::fs::copy on WASI by setting proper OpenOptions flags

When PR rust-lang/rust#147572 switched WASI to use Unix-style filesystem APIs, the open_to_and_set_permissions function for WASI was implemented to call OpenOptions::new().open() without setting any access mode flags.

This causes std::fs::copy to fail with the error:
"must specify at least one of read, write, or append access"

The fix is to explicitly set .write(true), .create(true), and .truncate(true) on the OpenOptions, matching the behavior of the non-WASI Unix implementation but without the permission handling that WASI doesn't support.

Minimal reproduction:
```rs
    fn main() {
        std::fs::write("/src.txt", b"test").unwrap();
        match std::fs::copy("/src.txt", "/dst.txt") {
            Ok(_) => println!("PASS: fs::copy works!"),
            Err(e) => println!("FAIL: {}", e),
        }
    }
```
    # Compile and run:
    rustc +nightly --target wasm32-wasip2 test.rs -o test.wasm
    wasmtime -S cli --dir . test.wasm

    # Before fix: FAIL: must specify at least one of read, write, or append access
    # After fix:  PASS: fs::copy works!

Note: The existing test library/std/src/fs/tests.rs::copy_file_ok would have caught this regression if the std test suite ran on WASI targets. Currently std tests don't compile for wasm32-wasip2 due to Unix-specific test code in library/std/src/sys/fd/unix/tests.rs.

Fixes the regression introduced in nightly-2025-12-10.

r? @alexcrichton