Python: Add background agent support to harness agent

[westey-m]

Motivation and Context

#6154

Description

• Add background agent support to harness agent

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

Adds opt-in background agent support to the Python harness agent so a harness-created agent can delegate work to background sub-agents via a BackgroundAgentsProvider.

Changes:

• Extend create_harness_agent() / _assemble_context_providers() to accept background_agents and background_agents_instructions.
• Automatically append BackgroundAgentsProvider when background_agents is provided and non-empty.
• Add unit tests covering provider inclusion/default behavior and custom instructions wiring.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
python/packages/core/agent_framework/_harness/_agent.py	Adds background_agents/background_agents_instructions parameters and wires BackgroundAgentsProvider into the harness context provider pipeline.
python/packages/core/tests/core/test_harness_agent.py	Adds tests verifying background-agent provider is (not) included by default, included when configured, and honors custom instructions.

[moonbox3]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework/_harness
_agent.py	73	4	94%	132, 342–343, 345
TOTAL	36868	4313	88%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
7389	34 💤	0 ❌	0 🔥	1m 57s ⏱️

[github-actions]

Automated Code Review

Reviewers: 4 | Confidence: 90% | Result: All clear

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

---

Automated review by westey-m's agents