Reject unknown rust-toolchain.toml keys

[Bortlesboat]

Reject unknown keys at both levels of rust-toolchain.toml deserialization.

This prevents misspellings such as channnel = "nightly" from being silently ignored and also rejects unsupported top-level sections. The TOML parser reports the unknown key and lists the supported fields, while valid and legacy toolchain files retain their existing behavior.

Fixes #2876.

Validation:

• RED/GREEN tests for an unknown [toolchain] key and an unknown top-level section
• all 13 config::tests parser tests pass with the test feature
• cargo clippy --all-targets --features test -- -D warnings
• cargo fmt --all -- --check
• git diff --check

I also attempted the broader --all-features Clippy command. On this Windows host its optional vendored-OpenSSL lane requires a perl executable and stopped while configuring OpenSSL; the repository's normal Windows feature set above is green.

[djc]

There already was a PR for this before you claimed it:

• fix: reject unknown toolchain file keys #4953