file_store: oversized entry length prefix panics Store::load instead of returning an error #2306

Describe the bug

EntryIter::next decodes each entry with bincode_options().deserialize_from(..), and bincode_options() (crates/file_store/src/lib.rs) sets no size limit. A length prefix read from the file is therefore passed straight to the buffer allocation. An entry declaring a String length of u64::MAX panics with capacity overflow inside Store::load; smaller but still oversized lengths attempt a huge allocation instead of returning StoreError::Bincode.

This affects loading a corrupted or hand-edited store file. #2258 appears to address this by bounding the frame allocation.

This issue was found by AI.

To Reproduce

Add crates/file_store/tests/test_oversized_len.rs and run cargo test -p bdk_file_store --test test_oversized_len:

use bdk_file_store::Store;
use std::collections::BTreeSet;

const MAGIC: &[u8] = b"bdk_test_magic";

#[test]
fn load_returns_error_on_oversized_length_prefix() {
    let dir = tempfile::tempdir().unwrap();
    let path = dir.path().join("db");

    let mut bytes = MAGIC.to_vec();
    bytes.push(1); // BTreeSet length: 1 element
    bytes.push(253); // bincode varint tag: u64 follows
    bytes.extend_from_slice(&u64::MAX.to_le_bytes()); // String length: u64::MAX
    std::fs::write(&path, &bytes).unwrap();

    let result = Store::<BTreeSet<String>>::load(MAGIC, &path);
    assert!(result.is_err(), "load should fail with an error, not panic");
}

Current output:

thread 'load_returns_error_on_oversized_length_prefix' panicked at library/alloc/src/raw_vec/mod.rs:28:5:
capacity overflow

Expected behavior

Loading a store file whose entry declares a length that cannot be satisfied should return an error rather than panic or attempt an unbounded allocation.