fix: run Clippy and the TODO check on the Windows leg too

[MaximeGaudin]

Summary

• The check job's Clippy step was guarded with if: matrix.os != 'windows-latest' and the TODO/stub check with if: runner.os != 'Windows', so the Windows matrix leg only ever ran cargo test — an unexplained coverage gap.
• Removed both guards so Clippy and scripts/check-no-todos.mjs run on all three OSes.

Why this is safe: the Windows leg already compiles the entire workspace (including the C build in aws-lc-sys) via cargo test --workspace, and the release workflow builds the Windows target the same way, so the only new work is the clippy lint pass over code that is already type-checked on Windows. The todo checker is a plain Node script with no POSIX-only APIs (CRLF is handled by its line splitting). No #[cfg(windows)] code exists in the workspace, so no platform-specific lint surface is introduced.

Fixes #184

Test plan

• YAML parses locally. Full Windows verification happens in the PR's own CI run (the Windows check leg now executes both steps).
• CI green on the PR — the Windows leg is the verification