fix: avoid incorrectly unchanged cargo workspace crates

[jolars]

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #2748

Summary

This PR fixes a manifest-mode bug where a root Rust release candidate (path:
".") could retain workspace member Cargo.toml updates generated by the Rust
strategy, causing member versions to be rewritten to the root version and
diverging from per-path manifest decisions.

What changed

• Updated CargoWorkspace plugin to drop pre-existing root-candidate updates
  targeting workspace member manifests, while keeping the root Cargo.toml
  update.
• Kept normal workspace/plugin-driven version propagation and Cargo.lock
  handling intact.

Tests

Added regression test:

• test/plugins/cargo-workspace.ts
• removes root rust strategy member updates outside workspace decisions

This test verifies root candidates no longer include unintended member
Cargo.toml bumps.