fix: resolve clippy --all-targets warnings in test code

[edochi]

Summary

• Remove unused mut on MdvsToml bindings in test code (build, search, config tests)
• Remove unused FieldTypeSerde and ScanConfig imports in update tests
• Replace 3.14 with 3.15 in storage tests to avoid clippy::approx_constant
• Update CLAUDE.md to document cargo clippy --all-targets as the standard lint command

Test plan

• cargo clippy --all-targets — clean
• cargo test — 315 passing
• cargo fmt — clean