build: update Rust to 1.99, install taplo from its release binary

[psteinroe]

The Format and Check Codegen jobs fail on every branch: moonrepo/setup-rust builds taplo-cli with cargo install without --locked, which now resolves uuid 1.27.0. That release needs rustc 1.89, and our toolchain was 1.88.

taplo from its release binary. Both jobs download the static taplo 0.10.0 release binary instead of building it. It's the same version, so formatting doesn't change, the jobs no longer compile taplo, and a future dependency release can't break them this way again.

Rust 1.99. rust-toolchain.toml moves from 1.88 to the current stable, 1.99, and flake.lock updates rust-overlay so the Nix shell knows it. The new clippy lints are fixed:

• Nested ifs are collapsed into let-chains, the style the codebase already uses. The generated pgls_configuration rules files change through the codegen templates in xtask/codegen, not by hand.
• Redundant references in format macros, same-type casts in the pgls_query FFI code, function-item casts in the backtrace helpers, and an unused lifetime in pgls_treesitter.
• Two test fixtures in pgls_diagnostics that rustc now reports as never constructed are only used by tests that are commented out. They are marked #[allow(dead_code)].