fix(file_store): reject an entry length prefix larger than the file

[wangzhengzhuo05]

Fixes #2306

What

EntryIter::next decoded each entry with bincode_options().deserialize_from(..) and no size
limit, so a length prefix read from the file was passed straight to the buffer allocation. A
store file whose entry declares a String length of u64::MAX panicked inside Store::load:

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

How

Each entry is now decoded with the deserializer bounded by the bytes remaining in the file,
multiplied by nine:

• bincode charges size_of::<u64>() (8 bytes) for every length literal it decodes, even when the
  varint encoding occupies a single byte on disk, and then charges the payload length again. A
  valid entry therefore charges at most 8 * bytes + bytes.
• Every length literal occupies at least one byte on disk, so the total charge can never exceed
  nine times the bytes remaining — the bound cannot reject a store that fits in the file.
• Any length prefix that cannot be satisfied now fails with bincode::ErrorKind::SizeLimit
  before an allocation is attempted, and the existing error arm maps that to
  StoreError::Bincode.
• remaining == 0 (end of file) short-circuits to the existing clean-EOF return instead of
  handing the deserializer a zero limit.

Only crates/file_store/src/entry_iter.rs changes; lib.rs and store.rs are untouched.

Tests

Added crates/file_store/tests/test_oversized_entry_len.rs:

• load_returns_error_on_oversized_length_prefix — the reproduction from the issue: load
  returns an error instead of panicking.
• valid_store_still_round_trips — create/append/load two changesets, so the bound is not
  tighter than a real store needs.

cargo test -p bdk_file_store                                 # 10 + 2 + 1 passed
cargo clippy -p bdk_file_store --all-targets -- -D warnings   # clean
cargo fmt --all -- --check                                    # clean

Mutation check: with this change reverted,
load_returns_error_on_oversized_length_prefix panics with capacity overflow; with the change
applied, it passes.

Note on #2258

#2258 (replace bincode with postcard) also bounds the frame allocation and rewrites this same
file, so it supersedes this fix if it lands first. This change is scoped to the current framing
and is independently correct and testable today; I am happy to rebase or drop it in favour of
#2258 if that is the preferred path.

Implemented with AI assistance (OpenCode + muse-spark 1.3); the reproduction, the full crate
test suite, clippy, rustfmt and the mutation check above were run locally and reviewed before
submission.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.85%. Comparing base (c6a6073) to head (6edf809).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2313      +/-   ##
==========================================
+ Coverage   78.84%   78.85%   +0.01%     
==========================================
  Files          31       31              
  Lines        6060     6073      +13     
  Branches      288      289       +1     
==========================================
+ Hits         4778     4789      +11     
- Misses       1203     1204       +1     
- Partials       79       80       +1     

Flag	Coverage Δ
rust	78.85% <100.00%> (+0.01%)	⬆️

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[nymius]

Hi @wangzhengzhuo05, bincode must be replaced because is unmaintained. postcard is the best candidate we have so far.
As this is the priority, this PR is not going to get merged before #2258, which would make this PR obsolete.
I recommend you to close this PR and focus on review of #2258 or look for other issues to fix in BDK, if you are up to it.
Thanks!