fix(qwen): complete ACP event handling

[anxkhn]

Related issue

Closes #4876

Related: #1638 (Qwen Stop button only; Hermes stays on that issue / #1643).

Summary

Qwen already runs through ACP, but several protocol surfaces never made it
into Omnigent.

This keeps the existing QwenExecutor and wires the missing events and
the standard model-config method.

• Stop sends ACP session/cancel after the session exists. SIGTERM is
  only the fallback before setup or when the notification fails.
• agent_thought_chunk becomes ReasoningChunk.
• tool_call / tool_call_update become ToolCallRequest /
  ToolCallComplete.
• /model uses session/set_config_option on the live session.
  session/new.model is no longer sent.
• The process manager no longer respawns Qwen when the model changes.
• An empty per-turn ExecutorConfig no longer drops HARNESS_QWEN_MODEL.

Hermes interrupt is out of scope. #1643 still covers that half of #1638.

Test Plan

• uv run --python 3.12 --group test --extra databricks --frozen pytest tests/inner/test_qwen_executor.py tests/runtime/test_process_manager.py -q → 97 passed
• New coverage for cancel, thought/tool events, session/set_config_option,
  configured-model fallback, and Qwen live-process /model.
• pre-commit on the changed files: ruff, format, pyrefly clean.

Demo

N/A. Backend harness event and lifecycle wiring. The existing Qwen ACP
session UI is unchanged except that Stop, reasoning, tools, and /model
now have something to show.

Type of change

• Bug fix
• Feature
• UI / frontend change
• Refactor / chore
• Docs
• Test / CI
• Breaking change

Test coverage

• Unit tests added / updated
• Integration tests added / updated
• E2E tests added / updated
• Manual verification completed
• Existing tests cover this change
• Not applicable

Coverage notes

Unit tests drive the ACP notification and RPC paths. A live signed-in
Qwen turn is the best human check: Stop mid-turn, watch reasoning and
tool cards, then /model without losing the session.

Changelog

Qwen Stop, reasoning, tool cards, and /model now follow the ACP session
instead of being dropped

[github-actions]

@anxkhn This PR is a Bug fix, Feature, or UI / frontend change but the Demo section is missing or only contains a placeholder.

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