doctor: share the per-language availability loop with list_supported_languages, and move presentation knowledge into adapters

[debugmcpdev]

Follow-ups from the #423 doctor command review — two structural improvements deferred from the initial implementation:

1. One shared per-entry availability probe

diagnoseLanguage (src/cli/commands/doctor/diagnose.ts) hand-mirrors the server's handleListSupportedLanguages per-entry loop (src/server.ts: disabled gate → installed gate → getFactory().catch → metadata-over-entry attach preference), with only computeModeAvailability actually shared. Doctor's "can never disagree with the server" guarantee currently rests on manual mirroring: the next change to the server's loop semantics lands in server.ts only and doctor silently diverges — the exact class of drift doctor exists to expose.

Extract one shared per-entry probe (natural home: src/utils/language-availability.ts) consumed by both handleListSupportedLanguages and diagnose, and add a parity test that runs both paths against the same fake registry and asserts identical modes.

2. Adapter-owned doctor presentation

presentLanguage and collectDoctorExtras (src/cli/commands/doctor/presenters.ts) hardcode ~20 stringly-typed validate().details keys plus language→package→export-name mappings in the CLI layer, duck-typed through Partial casts with silent fallbacks. Renaming a details key or an extras export in an adapter package compiles clean and passes all tests while doctor's output silently blanks.

Deeper fix: adapter-owned presentation — typed runtime/backend fields on AdapterMetadata, or an optional IAdapterFactory.describeToolchain() — so an adapter package owns its own doctor row and the CLI stops restating adapter internals. This also removes the "wire the doctor command" step from the new-adapter checklist in CLAUDE.md.

[debugmcpdev]

Part 1 landed in #437 (squash 9699ee2): probeLanguageEntry in src/utils/language-availability.ts is now the single per-entry availability loop, consumed by both list_supported_languages and doctor, with a parity test (tests/core/unit/server/server-doctor-parity.test.ts) fencing them together. Along the way it picked up attach-string normalization, fault-isolated getMetadata, warn-level breadcrumbs for unprobeable factories, parallel server-side probing, and doctor-side hang containment for wedged factory imports.

Remaining scope for this issue:

1. Adapter-owned doctor presentation (original part 2) — typed runtime/backend description fields on AdapterMetadata or an optional IAdapterFactory method, so presenters.ts stops restating ~20 stringly-typed details keys per adapter.
2. Route checkLaunchToolchain through the shared probe — it remains a second hand-rolled getFactory→validate→fail-open path in the same file (the launch gate for create_debug_session/start_debugging), which can drift from the shared loop the same way server/doctor used to.
3. Typed getFactory surface — server.ts still reaches the registry through an as unknown as duck-typed cast; a rename on the concrete AdapterRegistry would compile clean and silently degrade every language to fail-open. Related: AdapterRegistry.getFactory swallows loader errors (catch { return undefined }), so the probe's factoryLoadError diagnostic can only fire for registries that propagate errors — surfacing the underlying import failure needs a registry-level change.

[debugmcpdev]

All three remaining parts have landed:

Part 2 — adapter-owned doctor presentation (#442): optional IAdapterFactory.describeToolchain(validation, options?) on @debugmcp/shared; each adapter renders its own runtime/backend row from the validate() result it produced, behind a package-local details type alias — key renames are now compiler-checked inside the package. presenters.ts (the 19-key stringly-typed mapping, collectDoctorExtras, and the literal-specifier import switch) is deleted; the former extras probes live in their packages (dotnet parallel, cpp command-reuse); the "wire the doctor command" step is gone from the new-adapter checklist. Hardened per review: verdict computed from a pre-call snapshot (a plain-JS factory can't mutate its way past broken), withTimeout tolerates non-thenable returns, and probeWithinBudget keeps dotnet/cpp resolving with partial cells instead of a hard timeout blanking the row. Bug fixed en route: the js-debug cell no longer renders "vendored" when the payload is missing.

Part 3 — launch gate on the shared probe (#443): checkLaunchToolchain is a thin wrapper over probeLanguageEntry (synthetic entry neutralizes the disabled/notInstalled short-circuits — those are enforced upstream), so the gate's availability and reason text are literally computeModeAvailability's launch mode. A drift-fence matrix (checkLaunchToolchain ≡ probe.modes.launch across registry shapes) is the gate-side analogue of the doctor parity test.

Part 4 — typed getFactory surface (#443): IAdapterRegistry gains required listLanguages/listAvailableAdapters/getFactory/getFactoryMetadata and optional getFactoryResult; all four as unknown as cast sites are removed. getFactoryResult carries the loader's real error, so doctor now reports the actual import failure instead of "(the registry returned no factory)"; the remaining discovery-fallback swallows log warnings. AdapterManifestEntry disambiguates the loader-manifest shape from the factory-declared AdapterMetadata.

Deferred (all cheap, none blocking):

• Consolidate the three near-identical doctor fake-registry builders into a shared test util.
• Doctor wording for dynamicLoadingDisabled (production always enables dynamic loading, so it never surfaces today).
• Switch the [FEATURE] start_debugging should fail fast when the target adapter is known-unavailable, instead of reporting success #360 attach carve-out site (server.ts create_debug_session) from getFactory+getMetadata to getFactoryMetadata.
• Extend the isAdapterRegistry runtime guard to the new members.
• cpp doctor runs still spawn <compiler> --version twice (validate presence-check + banner probe) — pre-existing; fixing it means retaining the banner in validate, which is deliberately presence-only for the server hot path.