CI maintenance

[hildebrandmw]

Perform the following CI chores:

• Add feature tests to clippy and include the disk-index feature for diskann-benchmark.
• Pass -Dwarnings explicitly to the clippy jobs. Previously, this wasn't being propagated from build.rustflags, meaning that clippy lints weren't blocking check-in.
• Rewrite the no-default-features tests to execute serially instead of as multiple jobs. This reduces the CI cache used for this job from ~850 MB for the 9 tested crates to around 160 MB. The job takes a little longer, but still under 1 minute.

Finally, since clippy now correctly respects -Dwarnings, there were two lints that needed fixing: one invalid conversion, and one regarding the disallowed overlay method (which doesn't exist when default features are disabled).

[Copilot]

Pull request overview

Updates the GitHub Actions CI workflow to standardize which optional Rust features are exercised in CI and to expand lint coverage across configurations.

Changes:

• Introduces a DISKANN_FEATURES env var to centralize the explicit feature set used for “all-features”-style checks.
• Adds a new clippy-features job to run cargo clippy with the explicit feature set.
• Adjusts Swatinem/rust-cache configuration to use shared-key for several jobs and reuses DISKANN_FEATURES for the feature-enabled test job.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.28%. Comparing base (72fc467) to head (d4fa355).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #838      +/-   ##
==========================================
+ Coverage   89.10%   89.28%   +0.18%     
==========================================
  Files         443      442       -1     
  Lines       83361    83009     -352     
==========================================
- Hits        74277    74113     -164     
+ Misses       9084     8896     -188     

Flag	Coverage Δ
miri	89.28% <ø> (+0.18%)	⬆️
unittests	89.12% <ø> (+0.17%)	⬆️

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

Files with missing lines	Coverage Δ
diskann-tools/src/utils/search_disk_index.rs	0.00% <ø> (ø)

... and 5 files with indirect coverage changes

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[hildebrandmw]

Thanks @metajack - I learned way more than I anticipated about how the CI cache works.