Python: Move InMemory history provider injection to the first invocation

[TaoChenOSU]

Motivation and Context

Move the InMemoryHistoryProvider injection to when the agent is first invoked, similar to how a normal agent works. This is required by hosted agent as it doesn't need a history provider preconfigured.

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.

[moonbox3]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework/_workflows
_agent.py	360	71	80%	66, 74–80, 116–117, 210, 271, 284, 351, 362, 364, 424, 430, 440–441, 448, 450, 456, 518–519, 528, 535, 561, 594–596, 598, 600, 602, 607, 612, 659, 689, 706, 745–748, 754, 760, 764–765, 768–774, 778–779, 787, 848, 855, 861–862, 873, 905, 912, 933, 942, 946, 948–950, 957
TOTAL	27205	3185	88%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
5458	20 💤	0 ❌	0 🔥	1m 27s ⏱️

[Copilot]

Pull request overview

This PR changes when WorkflowAgent auto-injects InMemoryHistoryProvider, moving it from construction time to the first invocation so the workflow-wrapped agent behaves more like a “normal” agent lifecycle.

Changes:

• Removed default InMemoryHistoryProvider injection from WorkflowAgent.__init__.
• Added runtime injection of InMemoryHistoryProvider in _run_impl and _run_stream_impl when no “loading” HistoryProvider is detected.