feat: add Rust and Cargo doctor diagnostics

[dDevAhmed]

Summary

Implements CLI-019 by adding independent Rust and Cargo environment diagnostics to the default stellarforge doctor run.

Included

• rustc --version diagnostic through the existing safe subprocess adapter
• cargo --version diagnostic through the same adapter
• Rust compatibility enforcement at >=1.84.0, matching current Stellar smart-contract setup guidance
• Cargo availability and robust numeric-version parsing without inventing an unsupported minimum
• independent failure/remediation for missing, malformed, and incompatible tool states
• unit tests for present/absent/malformed Rust and Cargo outputs
• secret-leak prevention for failed subprocess execution
• built-CLI integration coverage updated to expect Node.js, npm, Git, Rust, and Cargo
• minor Changeset for new user-visible diagnostics

Security

• no shell interpolation
• only rustc --version and cargo --version are executed
• no rustup/toolchain/configuration inspection
• subprocess stderr/error details are not surfaced to users
• no new dependencies

Reference

Current Stellar smart-contract documentation states Rust 1.84.0 or newer is required for wasm32v1-none workflows.

Closes #22