Python: fix: bound background_agents_wait_for_first_completion with a timeout

[HUAN2022A]

Summary

background_agents_wait_for_first_completion could suspend the calling agent's run forever when a child task never completes. This PR adds an optional bounded timeout: a wait_timeout_seconds provider default (300s) and a timeout_seconds argument on the tool itself. None restores the previous unbounded behavior. When the wait elapses, the provider refreshes task state through its existing LOST-detection path and returns the current statuses to the model.

Fixes #7454

Validation

• Background-agents test file: 30/30 passed, including 3 new tests
• Related harness/loop/observability test files pass
• ruff check, ruff format, and pyright clean on the changed module

[Copilot]

Pull request overview

This PR prevents background_agents_wait_for_first_completion from potentially suspending a parent agent run indefinitely by adding a configurable timeout: a provider-level wait_timeout_seconds default and a per-call timeout_seconds override, plus new tests covering timeout behavior and defaulting.

Changes:

• Add wait_timeout_seconds to BackgroundAgentsProvider (default 300s) and apply it in background_agents_wait_for_first_completion.
• Add an optional timeout_seconds parameter to the wait tool and return a status summary when the wait times out.
• Add tests for timeout behavior, provider-default timeout behavior, and explicit timeout behavior when the task completes before the deadline.

Reviewed changes

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

File	Description
python/packages/core/agent_framework/_harness/_background_agents.py	Adds provider- and tool-level timeout support to the wait-for-completion tool, including a timeout return path that refreshes task state.
python/packages/core/tests/core/test_harness_background_agents.py	Adds new hanging-agent and timeout-focused tests to ensure waits are bounded and return status summaries.

Suppressed comments (1)

python/packages/core/agent_framework/_harness/_background_agents.py:415

• asyncio.wait requires a non-negative timeout (or None). With the new parameters, a negative timeout_seconds (or a negative provider default) will raise and fail the tool invocation rather than returning a normal error/result string. Validate and return an error message before calling asyncio.wait.

            effective_timeout = self._wait_timeout_seconds if timeout_seconds is None else timeout_seconds
            done, _ = await asyncio.wait(
                [t for _, t in waitable],
                return_when=asyncio.FIRST_COMPLETED,
                timeout=effective_timeout,