feat(engine): add available_backends() host-capability probe API

[Huzaifa Danish (huzaifa-d)]

📖 Description

Phase 2 of the backend-support-probe plan (design doc in #717): add the read-only available_backends() host-capability probe API.

available_backends() -> Vec<AvailableBackend> reports only the containment backends the current host can actually run, so a caller can pick a backend at startup without attempting an execution. It is deliberately separate from platform_support() (§7.1): that answers the narrower "which backends can mxc-sdk itself launch?" question, whereas this answers the broader host-capability question and additionally reports each backend''s effective isolation tier.

What''s included (new mxc_engine::probe module, re-exported from mxc_engine and the public mxc-sdk):

• AvailableBackend { backend: String, tier: Option<String> } — serializes to camelCase JSON ({"backend":"seatbelt"} / {"backend":"processcontainer","tier":"appcontainer-dacl"}); tier is omitted (never null) when the backend has no tier ladder. backend is a canonical Containment wire name; tier is a canonical IsolationTier::as_str() string.
• available_backends() — per-platform arms reusing the detectors from feat(engine): Rust host-backend detectors + available_backends() probe #725: macOS → seatbelt; Linux → bubblewrap/lxc; Windows → processcontainer (with effective tier) + windows_sandbox + wslc (feature-gated). An empty Vec is a normal result (unsupported platform, or Linux with neither bwrap nor lxc), not an error.
• select_tier() — a pure precedence fn (base-container → appcontainer-bfs → appcontainer-dacl floor), split from the host detectors so it''s unit-testable without a real Windows host or the tier2_bfs feature.

The named tier is a ceiling, not a guarantee: a real request can still land on a weaker tier when policy forces it (e.g. deniedPaths on a host without native deny support). The probe performs none of those policy-dependent checks.

Stacking / scope:

• 🥞 Stacked on feat(engine): Rust host-backend detectors + available_backends() probe #725 (user/modanish/port-host-detectors-to-rust) — this reuses the lxc/windows_sandbox detectors that land there. Review/merge feat(engine): Rust host-backend detectors + available_backends() probe #725 first; the diff here is just the probe module + two re-export lines.
• The four still-missing detectors (isolation_session, microvm, hyperlight) and the side-effect-free TypeScript-projection transport are out of scope, tracked as the §6 follow-ups.

🔗 References

Related to #717 (design doc / plan this implements Phase 2 of).

• feat(engine): Rust host-backend detectors + available_backends() probe #725 (base branch — the host-detector port this stacks on)

🔍 Validation

• cargo fmt --all -- --check — clean
• cargo clippy --all-targets -- -D warnings — clean on x86_64-pc-windows-msvc and x86_64-unknown-linux-gnu, and under features wslc and tier2_bfs
• cargo test -p mxc_engine — 25 passed (incl. 8 new probe tests: serde shape with tier omitted vs present, host + unconditional wire-name drift guards, canonical-tier drift guard, Windows processcontainer-always-with-tier, tier precedence across all combos, non-Windows processcontainer absence). mxc-sdk suites pass.

Reviewed by an independent adversarial pass and against the repo Rust conventions; the unconditional wire-name drift test and the exotic-target #[cfg] gate came out of that review.

✅ 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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[Huzaifa Danish (huzaifa-d)]

Folded into #725. This PR''s commits became an ancestor of its base branch (user/modanish/port-host-detectors-to-rust), so GitHub auto-marked it as merged — but the probe was not merged to main; it now ships as part of #725, which carries both the host detectors and the available_backends() probe API. No separate probe PR. Closing out here; the available-backends-probe-api branch is being deleted.