fix(runtime-host): import external sessions without a configured default model

[liuxiaocs7]

Summary

External-session import (Settings → 活动 → 导入任务) failed for every conversation with the generic banner "该对话无法转换或保存", regardless of size or source.

Root cause: resolveExternalSessionImportTarget resolved the imported task's model with a hardcoded { kind: 'default' } target, so it threw "No default Session model is configured" whenever the workspace had no default set — a legitimate state for a self-configured profile that still has usable connections (a default is only auto-set during onboarding bootstrap). The throw happened before the source was read, so it failed every import; at the IPC boundary it became a RuntimeHostOperationError the renderer redacted down to the generic fallback banner.

This resolves the import model by preferring the configured default but falling back to the first ready connection+model in the catalog, reusing the existing explicit-target readiness path (resolveExecutionConnection + authorizeConnectionModel) so disabled / retired / credential-less / non-chat candidates are skipped. Behavior is unchanged when a default is set and ready.

It also makes import errors actionable: because Electron strips the error code before the renderer sees it, Desktop Main now maps the intact Host operation code to typed IPC reasons (no_model, source_unreadable) so the page shows a specific banner instead of the redacted fallback. This also gives the oversized-rollout (> 64 MB) failure an honest "too large" message rather than the same generic text.

Fixes#4226

Verification

Ran locally (from the worktree):

• runtime-host — 53/53 pass (session-catalog-coordinator.test.ts, external-session-coordinator.test.ts), including 4 new cases: fallback when no default is set; skip an unready default and use the next ready connection; clean failure when no connection is usable; and the previously-uncovered resolveTarget-throws → importFailure branch. The 3 new coordinator tests fail without the fix.
• desktop main — 23/23 pass (import-tasks-settings-page.test.ts, runtime-host-external-sessions-ipc-main.test.ts), including 5 new cases: the code→reason mapping (no_model, source_unreadable, and rethrow for an unmapped code), and the two new banners rendered by the page.
• tsc (typecheck) clean for @maka/core, @maka/runtime-host, @maka/ui, and desktop build:main.
• biome check clean on all changed files.

Not run: the full-repo npm run build / npm run typecheck and knip. (This environment has a pre-existing, unrelated @maka/runtime type error — HttpsProxyAgent is not generic — that predates and is untouched by this change.)

User-visible change: the import error banner. New copy — zh importFailedNoModel: "没有可用的模型连接，无法为导入的任务选择模型。请先在设置 › 服务商中配置并启用一个模型后再导入。"; importFailedSourceUnreadable: "无法读取或转换该对话，它可能过大或已损坏。请检查来源后重试。" (English equivalents added). No layout change, so no screenshot; the two banners are asserted by the page test.

Root cause

See #4226 for the full trace. The redacted banner (generalizedErrorMessageChinese → importFailedFallback) hid the real error, so the failure was undiagnosable from the UI — the typed-reason mapping above is the durable fix for that.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Claude Code (claude-opus-4.8) diagnosed the root cause against real local data, implemented the fix and tests, and drafted this PR. The commit carries a Generated-by trailer.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No