ci: Sync Rust linting checks from bootc-dev/bootc

ci: Sync Rust linting checks from bootc-dev/bootc

Align with bootc's validate target:
- Make cargo clippy gating (was "non-gating") with the same lint config
  as bootc (-A clippy::all -D clippy::correctness -D clippy::suspicious
  -Dunused_imports -Ddead_code)
- Extend clippy to cover test crates (tests/inst, tests/bootc-integration,
  tests/xtask)
- Add cargo doc with -D warnings to catch rustdoc issues

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Xiaofeng Wang <henrywangxf@me.com>