fix(tasks): fall back to active provider when task default has no model

[prime-axiom]

Problem

getTaskDefaultProvider() returned an explicitly configured task default provider even when that provider had no enabled models. Since providers can now be created without selecting a model upfront (#87, 8824e19), such a provider starts background tasks (heartbeat, cronjobs, manual defaults) with an empty model.

The configured-provider branch short-circuited with if (resolved) return resolved, so the active-provider fallback below was never reached when the configured provider had no usable model.

Change

• Only honor an explicitly configured task provider when it can actually run a model (explicit model pin, or a non-empty enabledModels). Otherwise fall back to the active provider/model selection — matching the documented behavior ("Defaults to the currently active chat provider").
• Extract the resolution into a pure, dependency-injected helper resolveTaskDefaultProvider() so the logic is unit-testable; getTaskDefaultProvider() is now a thin wrapper.

No behavior change for the common paths (explicit provider:model, configured provider with models, active-provider default).

Testing

• New unit tests in runtime-composition.test.ts cover: explicit provider+model pin, configured provider with models (unchanged), configured provider with no enabled models (falls back to active), unresolvable provider, active default, no active model, and no active provider.
• npm run lint, npm run baseline:unit (1127), npm run baseline:api (198), npm run build — all green.

[prime-axiom]

Self-review (cannot self-approve via GitHub). Verified against the full checklist:

• Scope: single focused fix, no unrelated changes.
• Correctness: configured task providers with no enabled models previously short-circuited (if (resolved) return resolved) before the active-provider fallback, starting background tasks with an empty model. Fix only honors a configured provider when it can run a model; otherwise falls back to the active provider/model. Common paths unchanged.
• Tests: 7 new unit tests including a regression for the empty-enabledModels case (fails on the old behavior).
• Build/lint/tests: npm run lint, npm run baseline:unit (1127 passed), npm run baseline:api (198 passed), npm run build — all green.
• Docs: aligns code with existing documented behavior ('Defaults to the currently active chat provider'); no doc change needed.

Ready for merge from my side.