[FEATURE] Add plugins Parameter to Agent

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Overview

Add a new plugins parameter to the Agent class that accepts a list of Plugin instances for extending agent functionality.

Parent Issue: #1636

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Problem Statement

Currently, high-level features are added via the hooks parameter using HookProvider. A dedicated plugins parameter provides clearer semantics for composable agent extensions.

Proposed Solution

Add plugins: list[Plugin] | None = None parameter to Agent.__init__ that initializes plugins after the agent is constructed.

Implementation Requirements

Agent.init Changes

def __init__(
    self,
    # ... existing parameters ...
    plugins: list[Plugin] | None = None,
    hooks: list[HookProvider] | None = None,  # Keep for backwards compatibility
):
    # ... existing initialization ...
    
    # Initialize plugins after agent is fully constructed
    if plugins:
        for plugin in plugins:
            if inspect.iscoroutinefunction(plugin.init_plugin):
                # Handle async init_plugin
                run_async(lambda p=plugin: p.init_plugin(self))
            else:
                plugin.init_plugin(self)
    
    # Fire AgentInitializedEvent after plugins are initialized
    self.hooks.invoke_callbacks(AgentInitializedEvent(agent=self))

Plugin Initialization Order

1. Built-in components initialized (model, tools, hooks registry)
2. HookProviders registered (from hooks parameter - deprecated)
3. Plugins initialized (from plugins parameter)
4. AgentInitializedEvent fired

Files to Modify

• src/strands/agent/agent.py - Add plugins parameter and initialization logic
• tests/strands/agent/test_agent.py - Add tests for plugins parameter

Example Usage

from strands import Agent
from strands.hooks import Plugin

class LoggingPlugin:
    name = "logging"
    
    def init_plugin(self, agent):
        agent.add_hook(BeforeModelCallEvent, lambda e: print("Model call starting"))

agent = Agent(
    plugins=[
        LoggingPlugin(),
        SkillsPlugin(),
    ]
)

Acceptance Criteria

• plugins parameter added to Agent.init
• Plugins are initialized with the agent instance after construction
• Both sync and async init_plugin methods are handled correctly
• Plugin initialization happens before AgentInitializedEvent is fired
• Unit tests cover plugin initialization with various scenarios
• Docstrings updated for Agent class

Dependencies

• Depends on: Plugin Protocol Definition (sub-issue 1636.1)