fix(rust): satisfy current clippy lints

The current toolchain promoted two test-only patterns to hard errors under
`-D warnings`: integer-to-pointer casts used as non-null dictionary-handle
sentinels, and a repeat/take construction that Clippy now replaces with
`repeat_n`. The casts never represented addresses and were only placeholders
for ownership fields that the cleanup functions must clear. Use the standard
dangling-pointer constructor for those sentinels and the equivalent concise
iterator for the generated test input. Runtime behavior and the ABI are
unchanged, while the repository's Rust lint gate is warning-free again.

Test Plan:
- `cargo clippy --manifest-path rust/Cargo.toml -- -D warnings` -- passed
- `cargo clippy --manifest-path rust/Cargo.toml --benches -- -D warnings` -- passed
- `cargo clippy --manifest-path rust/Cargo.toml --tests -- -D warnings` -- passed
- `cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets -- -D warnings` -- passed
- `cargo +nightly fmt --manifest-path rust/Cargo.toml` -- passed
- `cargo +nightly fmt --manifest-path rust/cli/Cargo.toml` -- passed

rust/src/fileio_backend.rs

@@ -1049,7 +1049,7 @@ mod tests {
         let mapped = unsafe { std::slice::from_raw_parts(buffer.cast::<u8>(), mapped_size) };
         assert_eq!(mapped, payload);
 
-        let mut dict_handle = 1usize as *mut c_void;
+        let mut dict_handle = ptr::dangling_mut::<c_void>();
         unsafe {
             FIO_rust_freeDict(
                 FIO_MMAP_DICT,
@@ -1074,7 +1074,7 @@ mod tests {
         assert!(!allocation.is_null());
         let mut buffer = allocation;
         let mut buffer_size = 16;
-        let mut dict_handle = 1usize as *mut c_void;
+        let mut dict_handle = ptr::dangling_mut::<c_void>();
 
         unsafe {
             FIO_rust_freeDict(

rust/src/zstd_compress.rs

@@ -4573,7 +4573,7 @@ mod tests {
     #[test]
     fn one_shot_promotes_nonfirst_rle_blocks() {
         let mut input = vec![b'B'; 256 * 1024 - 2];
-        input.extend(std::iter::repeat(b'A').take(100 * 1024));
+        input.extend(std::iter::repeat_n(b'A', 100 * 1024));
 
         let compressed = compress_input(&input, 1);
         assert!(compressed.len() <= 46);