ci: treat rust-toolchain changes as Rust source changes (#2009)

The change-detection paths filter did not list rust-toolchain.toml, so a
toolchain-only pull request reported no backend/core/tauri change and every
Rust job was skipped. PR #2003 bumped the toolchain to 1.98.0, went green in
48s without compiling anything, and was auto-merged; the new clippy lints only
surfaced on the develop push, after the merge.

Add rust-toolchain* to the backend, core, and tauri filters so a toolchain
bump runs rust-format, lint-core, test-core, lint-tauri, and test-tauri before
merge.