[WSLC] Preserve backend failure classification across the state-aware daemon protocol

[Soham Das (SohamDas2021)]

📖 Description

A host that cannot run WSLc now returns backend_unavailable to state-aware callers, and a policy the backend refuses returns policy_validation. Both previously arrived as backend_error, so the state-aware and one-shot surfaces disagreed about the same host condition and a caller had to string-match the message to tell an unusable host from a rejected policy.

The daemon's step helpers already attributed each failure to a phase, but the daemon threw that away when it reduced the step result to a bare message before replying over its named pipe. Every failure — unusable host, rejected policy, busy container — arrived flattened. The classification now survives the pipe, so all four non-exec phases (provision / start / stop / deprovision) report the same code the one-shot surface reports.

🔗 References

Resolves #1167. Split out of #843 (defect 4); originally raised in review on #1045. Typed backend errors came from #794.

🔍 Validation

• cargo fmt --all -- --check and cargo clippy --all-targets -- -D warnings, clean — including the feature-gated consumers (wxc, mxc_engine, mxc_ffi with --features wslc).
• cargo test --workspace, plus new unit tests covering the classification at each seam: phase → error kind in the daemon, kind → wire code on the client, and an unrecognized kind decoding to the catch-all.
• On a real WSL2 host: the #[ignore]d full_lifecycle_over_pipe test, which drives provision → start → exec → stop → deprovision through the daemon over a live named pipe. This is the regression check that unit tests cannot reach.
• E2E harness run_wslc_state_aware_tests.ps1: 76/77.

✅ 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 (see docs/pull-requests.md)

📋 Issue Type

• Bug fix
• Feature
• Task

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Copilot review overview

🟢 Approval recommended

The focused implementation preserves compatibility and includes appropriate seam-level coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Preserves WSLc failure classifications across the state-aware daemon boundary.

Changes:

• Adds typed unavailable/rejected daemon errors.
• Maps them to existing SDK error codes.
• Provides forward-compatible unknown-kind handling and tests.

File	Description
session_manager.rs	Preserves step failure phases.
state_aware.rs	Maps daemon kinds to SDK errors.
daemon_protocol.rs	Extends error kinds compatibly.
wslc-state-aware.md	Documents error mapping and compatibility.

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[Branden Bonaby (bbonaby)]

I'm wondering if we'll one day be able to move this daemon to be the mxc-daemon rather than wslc specific. I guess that only really makes sense if we have more than one using it.

I say this assuming there is no way to make wslc state aware completely in proc right?