feat(rust) - Add "none" value for Rust feature "components"

[Venkumahanti Subhankar (V-Subhankar-infy)]

Issue

Resolves #1716 — no supported way to opt out of the default Rust components
(rust-analyzer, rust-src, rustfmt, clippy).

Solution

Add an explicit "none" value for the components option. When set to none,
the component-install loop is skipped. Unset, empty, and custom-list values keep
their existing behavior. Includes version bump (1.5.1 -> 1.6.0), updated
docs/proposals, and a new rust_with_none_components test scenario.

Impact

No breaking changes — only the previously-unhandled literal none changes
behavior. none skips components beyond the selected rustup profile; under the
default minimal profile this means none of the four defaults are installed.

[Copilot]

🟡 Changes recommended

The current components="none" handling is brittle around whitespace/mixed values (can attempt to install a literal none component), and the new test script includes an unused helper that should be cleaned up.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds an explicit components: "none" option to the Rust feature so users can opt out of installing the default component set beyond the selected rustup profile, and updates metadata/docs/tests accordingly.

Changes:

• Treat literal components="none" as a sentinel to skip the rustup component-install loop.
• Bump Rust feature version to 1.6.0 and update feature docs/proposals to document/offer none.
• Add a new rust test scenario validating that default components are not installed when components="none".

File summaries

File	Description
test/rust/scenarios.json	Adds a new rust_with_none_components test scenario entry.
test/rust/rust_with_none_components.sh	Adds a new test script asserting default Rust components are not installed when components=none.
src/rust/README.md	Documents components='none' behavior in the options table.
src/rust/install.sh	Implements the none sentinel to skip component installation.
src/rust/devcontainer-feature.json	Bumps feature version, updates components description, and adds none to proposals.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 2
• Review effort level: Lite

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[Copilot]

🟢 Approval recommended

The implementation preserves existing behavior, validates conflicting values, and includes focused coverage and documentation.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0 new
• Review effort level: Balanced