fix(wslc): clarify WSL runtime prerequisite

[huzaifa-d]

📖 Description

Clarifies the WSLC error when WSL is too old. The error now points users to WSL 2.8.1+ and wsl --update; the getting-started guide is aligned.

🔗 References

Resolves #553

🔍 Validation

• cargo fmt --all -- --check
• cargo test -p wslc_common (45 passed)

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)
• If this PR changes Cargo.lock, the dependency-feed-check check passes

📋 Issue Type

• Bug fix
• Feature
• Task

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[Copilot]

Pull request overview

This PR clarifies WSLC prerequisite failures when WslcCanRun reports missing components, aligning runtime requirements (WSL runtime package minimum) with the getting-started documentation and adding a regression test for the new message.

Changes:

• Add a helper to produce a more specific WSLC prerequisite error when WslcCanRun reports WslPackage missing, and add a unit test covering the message contents.
• Update the WSLC getting-started guide to consistently document the minimum WSL runtime version (2.8.0+) and clarify that wslcsdk.dll is a separate dependency.

Show a summary per file

File	Description
src/backends/wslc/common/src/wsl_container_runner.rs	Improves prerequisite failure messaging from WslcCanRun and adds a unit test for the updated message.
docs/wsl/wsl-container-getting-started.md	Updates prerequisite and troubleshooting documentation to consistently state the WSLC minimum WSL runtime version and separate SDK DLL requirement.

Review details

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

[MGudgin]

WslcComponentFlags is a fieldless repr(u32) enum, but wslcsdk.h declares it as combinable flags. WslcCanRun can therefore return 3 when both components are missing, which is not a valid Rust enum discriminant and causes undefined behavior when read here. Please represent this FFI type as a transparent integer-backed bitmask and add coverage for combined values.

[MGudgin]

This troubleshooting row keys on "WSLC runtime not available", but the runner now emits "WSLC runtime unavailable" (src/backends/wslc/common/src/wsl_container_runner.rs:360). Since aligning the message and the docs is the whole point of this PR, this row now points at a string the code no longer produces, so a user who copies the actual error and searches the guide won't find it. Please change the row key to "WSLC runtime unavailable" so it matches the emitted text verbatim.

While you're standardizing on the new wording, the design doc docs/wsl/wsl-container-support-plan.md (lines 239 and 278) still describes this condition as "WSLC runtime is not available". It's outside this PR's diff and only an internal planning doc, so it's low priority, but updating it in the same pass would keep the terminology consistent across the repo.

[MGudgin]

The remedy here is "Install WSL 2.8.1 or newer and run wsl --update", but WslcCanRun can report VirtualMachinePlatform as missing, and updating the WSL package does not enable that Windows optional feature. A user who already has current WSL but no Virtual Machine Platform will run wsl --update, get the same failure, and have no path forward. Consider branching the guidance on the missing flag: keep the WSL version/update advice for the WslPackage case, and for the VirtualMachinePlatform case tell the user to enable that Windows feature (e.g. via Optional Features / DISM). The missing components are already available in the "missing" value, so the message can be specific about which one failed.