feat(hooks): add context reduction events for streaming status

[strands-agent]

Description

Add BeforeContextReductionEvent and AfterContextReductionEvent hooks that fire when the agent reduces context due to ContextWindowOverflowException.

This enables users to:

• Notify users that context reduction is starting/completing
• Display UI feedback during long conversations
• Monitor context window usage patterns
• Implement custom cleanup before messages are removed

Events Added

BeforeContextReductionEvent

• Fires before reduce_context() is called
• Attributes: exception, message_count

AfterContextReductionEvent

• Fires after reduce_context() completes
• Attributes: original_message_count, new_message_count, removed_count
• Uses reverse callback ordering (consistent with other "After" events)

Usage Example

import asyncio
from strands import Agent
from strands.hooks import BeforeContextReductionEvent, AfterContextReductionEvent

agent = Agent(model=...)

@agent.hooks.on(BeforeContextReductionEvent)
async def on_reduction_start(event):
    print(f"⏳ Context reduction starting ({event.message_count} messages)")

@agent.hooks.on(AfterContextReductionEvent)
async def on_reduction_complete(event):
    print(f"✅ Context reduced: {event.original_message_count} → {event.new_message_count} ({event.removed_count} removed)")

Related Issues

Closes #1511

Documentation PR

No documentation changes required - follows existing hook event patterns.

Type of Change

New feature

Testing

• Added unit tests for both new event classes
• Verified existing hook tests still pass (30/30 tests)
• I ran hatch fmt (formatter + linter)

Checklist

• I have read the CONTRIBUTING document
• I have added tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly (N/A - follows existing patterns)
• My changes generate no new warnings
• Any dependent changes have been merged and published