fix(aws-strands): preserve plugins on per-thread agents

[2830500285]

Fixes #1933

Summary

• add an explicit plugins option to the Python StrandsAgent wrapper
• forward plugin providers to each per-thread Strands agent when the installed Strands version accepts plugins
• fall back to _plugin_registry.add_and_init(...) when available, and warn instead of silently dropping plugins otherwise
• add regression coverage for kwarg forwarding, per-thread list isolation, falsy plugin inputs, and registry fallback

Validation

• uv run pytest tests/test_template_plugins_preservation.py tests/test_template_agent_propagation.py tests/test_template_hooks_preservation.py -q
• uv run pytest -q
• uv run python -m compileall -q src tests/test_template_plugins_preservation.py tests/test_template_agent_propagation.py
• git diff --check
• uv run ruff format --check tests/test_template_plugins_preservation.py && uv run ruff check tests/test_template_plugins_preservation.py

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[tylerslaton]

Thanks for tackling this. The explicit plugins path is a reasonable direction, but I think this still needs changes before it can safely close #1933.

The current tests prove that an explicit kwarg is forwarded through a mocked core, but they do not cover the linked issue's actual shape: a template Strands Agent that was already constructed with a plugin such as AgentSkills, then wrapped as StrandsAgent(agent=agent, name="...").

The main risk is around tool registration. This PR forwards the plugin list while still passing tools=self._tools to each per-thread Strands agent. For the issue's concrete shape, plugin-provided tools may already be present in agent.tool_registry.registry.values(). Current Strands processes explicit tools before it initializes plugins, and duplicate tool names raise, so AgentSkills can end up double-registering the skills tool if callers pass the same plugin to the wrapper.

I think this needs a regression test with a real plugin that registers both a tool and a hook, ideally AgentSkills or a small test plugin, and the fix should either recover plugins from the template or filter plugin-provided tools out of self._tools before forwarding plugins.

[BenTaylorDev]

Closing in favor of #2141, which addresses the same aws-strands per-thread plugin preservation (same files) and has already been reviewed/approved. Let's land it there. Thanks @2830500285 — appreciate the fix.