[BREAKING] Python: Change background-agent loop helper to resolve the provider from the agent

[westey-m]

Motivation & Context

The harness exposes todos_remaining / todos_remaining_message loop helpers that drive an
AgentLoopMiddleware until the agent's todos are complete. The equivalent for background work was
incomplete: there was a background_tasks_running(provider) check that required the caller to pass
the BackgroundAgentsProvider explicitly, and there was no companion next_message helper.

That explicit-provider shape does not fit create_harness_agent, which builds its context providers
internally — callers wiring loop_should_continue / loop_next_message do not have a handle to
the BackgroundAgentsProvider. Resolving the provider directly from the running agent
(agent.context_providers) is the better design and matches how todos_remaining already works.

Description & Review Guide

• What are the major changes?

  • Refactored background_tasks_running to resolve the BackgroundAgentsProvider from the
    running agent (via _resolve_context_provider) instead of taking it as an argument — so it works
    in a harness agent where the caller has no access to the provider. New signature is
    background_tasks_running(); the predicate reads session/agent and loops while any persisted
    task is RUNNING.
  • Added background_tasks_running_message, a next_message helper mirroring
    todos_remaining_message: it lists the still-running tasks (#<id> (<agent>): <description>) and
    reminds the agent to wait for them to finish and retrieve their results, returning None when the
    session/agent/provider is unavailable or nothing is running.
  • A shared internal _running_background_tasks(session, agent) backs both helpers.
  • Exported the new helper, updated the module docstring, and documented both helpers in the core
    AGENTS.md.

• What is the impact of these changes?

  • The two helpers can now be wired straight into create_harness_agent's
    loop_should_continue / loop_next_message, consistent with the todos pair.
  • background_tasks_running is an experimental harness helper; its signature changed from
    background_tasks_running(provider) to background_tasks_running(). Callers now pass the agent
    via the standard loop kwargs rather than the provider.

• What do you want reviewers to focus on?

  • The resolve-from-agent approach and the wording/format of the next_message reminder.

Related Issue

Fixes #6734

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[github-actions]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework/_harness
_loop.py	279	7	97%	482, 490, 565, 639, 682, 757, 913
TOTAL	42568	5086	88%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
8327	37 💤	0 ❌	0 🔥	2m 20s ⏱️

[Copilot]

Pull request overview

Adds Python harness loop helpers for background-agent tasks that match the existing todo-loop helper pattern by resolving BackgroundAgentsProvider from the running agent, enabling direct use with create_harness_agent loop hooks.

Changes:

• Refactors background_tasks_running to resolve the provider from agent.context_providers (signature becomes background_tasks_running()).
• Adds background_tasks_running_message to generate a next_message nudge listing still-running background tasks.
• Updates exports, docs, and adds/updates unit tests to cover the new behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
python/packages/core/agent_framework/_harness/_loop.py	Refactors the background-task loop predicate and introduces a companion next_message helper backed by a shared _running_background_tasks function.
python/packages/core/agent_framework/init.py	Exports background_tasks_running_message from the top-level package API.
python/packages/core/AGENTS.md	Documents the new helper pair and updated behavior/signature.
python/packages/core/tests/core/test_harness_loop.py	Updates existing tests for the new predicate signature and adds coverage for the new background_tasks_running_message helper.

[github-actions]

Automated Code Review

Reviewers: 5 | Confidence: 93% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Failure Modes, Design Approach

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Automated review by westey-m's agents