style: restore rustfmt on two sites from #1644

[justinchuby]

main is red at 843b0bf7 on cargo fmt --all -- --check (exit 1, 2 diffs),
which reddens Fast (Linux x86_64) and Rust quality. Both sites came in with
#1644:

crates/onnx-genai-engine/src/native_decode/mod.rs:1115
snapshot_recurrent_state_public signature folded across three lines;
it fits on one at 98 columns.

crates/onnx-genai-engine/src/native_decode/tests.rs:1412
spec.rewind(base_len).expect(...) on one line at 62 columns; rustfmt's
default use_small_heuristics breaks a chain over 60.

Whitespace only -- git diff -w is empty.

Verified under the pinned toolchain (rustfmt 1.9.0-stable (88d9e12ae1 2026-08-18), resolved from rust-toolchain.toml, no explicit rustup run):

cargo fmt --all -- --check 1 -> 0
clippy -p onnx-genai-engine -F native-backend --all-targets 0 (unchanged)

Reproduced on a clean origin/main worktree first, so the diffs are #1644's
and not inherited from my tree.

One observation, since #1620 pinned the toolchain specifically to stop this:
this is not toolchain drift. Both directions here are what 1.98.0's rustfmt
produces from a default config, and the pin is being honoured -- the code
simply was never run through cargo fmt. The pin removed the class where two
contributors format the same file two ways; it cannot do anything about code
that no formatter has touched. That is a merge-gating question, not a
toolchain one: #1644 merged with Rust quality red.

Co-authored-by: Copilot App 223556219+Copilot@users.noreply.github.com
Copilot-Session: 08760f2f-160f-41e5-828d-9d9b6045c00d

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.05%. Comparing base (843b0bf) to head (52ed91e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1646      +/-   ##
==========================================
- Coverage   81.73%   81.05%   -0.69%     
==========================================
  Files         384      384              
  Lines      179999   180260     +261     
  Branches   179999   180260     +261     
==========================================
- Hits       147127   146111    -1016     
- Misses      27933    29204    +1271     
- Partials     4939     4945       +6     

Flag	Coverage Δ
cli-ort-linux	82.60% <ø> (ø)
cli-ort-windows	82.20% <ø> (ø)
mlas	85.05% <ø> (-0.04%)	⬇️
offline	80.93% <ø> (-0.72%)	⬇️

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see 16 files with indirect coverage changes

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