Connector healthcheckPath + classified test errors (#3)

[keysersoft]

Problem

`POST /api/connectors/:id/test` always hit `baseUrl + '/'`. APIs without a root handler returned 404 and the UI flashed red even when the connector was fine. We also couldn't distinguish:

• wrong path (404)
• wrong credentials (401/403)
• network unreachable (DNS/SSRF/timeout)

They all surfaced as the same generic failure.

Changes

Schema (additive migration)

• `Connector.healthcheckPath String?` — the path used by Test Connection. Defaults to `/` when null.

Parser

• New `OpenApiParser.parseSpec` / `parseSpecFromUrl` return `{ tools, healthcheckPath? }`. The legacy `parse()` / `parseFromUrl()` are now thin wrappers preserving their old shape for any external callers.
• `detectHealthcheckPath` priority: `/health`, `/healthz`, `/_health`, `/ping`, `/status` → first non-parametric GET with no required params → `undefined`.

Service

• `testConnection()` uses `connector.healthcheckPath` when set and returns a `kind` classification:
  • `ok` — 2xx
  • `auth_failed` — 401/403 (handshake reached the API)
  • `not_found` — 404 (path doesn't exist; hint to set healthcheckPath)
  • `unreachable` — DNS, ECONNREFUSED, SSRF guard, timeout
  • `unsupported` — connector type with no test impl
  • `error` — anything else (5xx, parse error)

Controller

• Both import flows (`POST /import-spec` and `POST /import` with `source: 'openapi'`) write the auto-detected `healthcheckPath` when the user hasn't already set one. Re-import preserves a user's explicit setting.
• `CreateConnectorDto` / `UpdateConnectorDto` expose the field.

Frontend

• Connector detail page: edit form gets a "Healthcheck path" input with explanatory hint (REST connectors only).
• Test result banner colours/labels by kind: yellow `Auth rejected:` for 401/403; red `Not found:`/`Unreachable:`/`Error:` otherwise. Successful tests stay green.
• `api.ts` client typed for the new response shape.

Test plan

• 4 new unit tests for `detectHealthcheckPath` (preferred path, fallback chain, first GET, no eligible GET).
• `tsc --noEmit` exit 0 (backend + frontend).
• `npm test` → 624 passed across 25 suites.
• After deploy: import a spec with `/health` → check Connector.healthcheckPath populated. Hit Test → expect `kind: 'ok'`.
• After deploy: change healthcheckPath to `/does-not-exist` → Test should return `kind: 'not_found'` with the hint.