addresses #2687 item 5 and item 6

Draft sequencing note: this PR is intended to be finalized after #2769 (items 1/3/4) and #2778 (item 2) land, then rebased onto that updated main. The implementation and audit below describe the post-merge target shape, not a compatibility layer for multiple intermediate states.

Summary

• worker.py: introduce a typed DeerFlowRuntimeContext, annotate runtime helpers, keep the runtime cast with the [refactor] follow-ups deferred from #2666 (app_config threading) #2687 TODO marker.
• task_tool.py: read runtime app config through the new typed runtime context instead of an untyped cast.
• agent.py: require middleware construction to receive app_config explicitly, removing the ambient get_app_config() fallback from both _build_middlewares and _create_summarization_middleware.
• client.py: thread DeerFlowClient's preloaded self._app_config into _build_middlewares for the embedded-client path.
• tests: cover explicit summarization config threading, bare helper rejection, and embedded-client middleware config propagation.

Invariant Audit (item 6)

This audit follows the config-threading direction established by the preceding #2687 PRs: config should be resolved at composition roots and then passed explicitly through runtime/middleware construction.

• _build_middlewares: now requires app_config explicitly and passes it to _create_summarization_middleware — safe.
• make_lead_agent → _make_lead_agent: passes resolved_app_config explicitly to _build_middlewares — safe.
• DeerFlowClient._ensure_agent: DeerFlowClient.__init__ preloads config, and _ensure_agent now passes self._app_config into _build_middlewares — safe.
• Direct test invocations: previously omitted app_config; those tests now pass an explicit AppConfig or assert the bare helper call is invalid.

Verdict: the explicit-config invariant is enforced in the target post-#2769/#2778 state. Summarization middleware construction has a single config path, matching item 2's removal of duplicate config sources instead of preserving ambient fallbacks.

File Summary

• worker.py: typed runtime context helpers and documented cast boundary.
• task_tool.py: typed access to runtime config.
• agent.py: single explicit app_config path for middleware construction.
• client.py: embedded client passes its preloaded app config into middleware construction.
• test_lead_agent_model_resolution.py / test_client.py: regression coverage for explicit config threading and no bare helper calls.

Merge Plan

• Keep this PR as draft until fix(subagents): follow up app_config threading for #2687 #2769 and refactor(config): remove ACP singleton, expose acp_agents on AppConfig #2778 are merged.
• Rebase fix(runtime): type runtime context and confirm summarization invariant (#2687) #2779 onto the updated main after those PRs land.
• Re-run backend make lint and make test after the rebase before marking ready for review.