fix(cargo): rewrite toolchain selectors

[Sakshamm-Goyal]

Summary

• recognize Cargo rustup +toolchain selectors before supported subcommands
• preserve selectors while routing through the existing rtk cargo filter
• cover stable, nightly, version-pinned, and beta selectors

Closes #3387.

Validation

• cargo +1.91.0 fmt --all --check
• cargo +1.91.0 clippy --all-targets
• cargo +1.91.0 test
• cargo +1.91.0 test -- --ignored
• manual: target/debug/rtk hook check 'cargo +stable check'

AI assistance

Developed with OpenAI Codex assistance; I reviewed and validated the final diff.

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