fix(ci): build clean under Rust 1.99's clippy

[dekobon]

Why

CI installs floating stable, which became Rust 1.99.0 on 2026-09-28.
Two diagnostics new in that release turn every -D warnings job red —
clippy, the three test legs, coverage, chain audit, the whole
features matrix and lint — on main and on every PR against it.
The 21 failing checks on the seven most recent Dependabot PRs were this,
not the dependency bumps.

What

• clippy::assert_is_empty (pedantic, so at warn via
  [workspace.lints]): 27 test assertions across 17 files move from
  assert!(x.is_empty()) to assert_eq! / assert_ne! against a typed
  empty array, so a failure prints the offending contents. Where a
  serde_json::Value was being unwrapped to an array first, the pair
  collapses to one assert_eq!(v[..], json!([])), which also prints the
  value when it is not an array at all.
• semicolon_in_expressions_from_non_local_macros (future-incompat,
  -D warnings promotes it): fires on every closure-position
  insta::assert_json_snapshot! because insta 1.48's macro ends in a
  stray ;. insta 1.49.0 drops it. Cargo.lock takes 1.49.0 and both
  dev-dependency floors move 1.29.0 → 1.49.0, so a fresh resolve
  cannot pick the old macro back up.
• CHANGELOG.md entry under Unreleased / Fixed.

Test code, a dev-dependency and the changelog only; nothing in the
published library changes.

Verification

• make pre-commit → BCA_GATE: pass (gate=pre-commit) on Rust 1.99.0.
• cargo clippy --workspace --all-targets -- -D warnings clean with and
  without --all-features; the workspace-excluded enums, fuzz and
  xtask crates were already clean.
• cargo metadata --locked resolves on stable 1.99 and nightly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.02%. Comparing base (0efc41a) to head (6d102cd).

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #1517      +/-   ##
===========================================
- Coverage   100.00%   98.02%   -1.98%     
===========================================
  Files            5      359     +354     
  Lines          441    96210   +95769     
  Branches        10    95779   +95769     
===========================================
+ Hits           441    94312   +93871     
- Misses           0     1214    +1214     
- Partials         0      684     +684     

Flag	Coverage Δ
python	100.00% <ø> (ø)
rust	98.01% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
big-code-analysis-ast/src/langs.rs	95.26% <100.00%> (ø)
big-code-analysis-cli/src/check_format.rs	95.89% <100.00%> (ø)
src/output/sarif.rs	98.79% <100.00%> (ø)
src/suppression.rs	96.87% <100.00%> (ø)

... and 350 files with indirect coverage changes

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