feat(plugins): improve plugin creation devex with @hook and @tool decorators

[Unshure]

Motivation

Currently, plugin authors must manually register hooks in their init_plugin() method, which is verbose and error-prone:

class MyPlugin:
    name = "my-plugin"
    
    def init_plugin(self, agent: Agent) -> None:
        agent.add_hook(self.log_call, BeforeModelCallEvent)

This PR enables declarative hook registration using a @hook decorator, making plugin development more intuitive and reducing boilerplate:

class MyPlugin(Plugin):
    name = "my-plugin"

    @hook
    def log_call(self, event: BeforeModelCallEvent):
        print(event)

    @tool
    def printer(self, log: str):
        print(log)
        return "Printed log"

Resolves: #1739

Public API Changes

New @hook decorator

The @hook decorator marks methods for automatic registration:

from strands.plugins import Plugin, hook
from strands.hooks import BeforeModelCallEvent, AfterModelCallEvent

class MyPlugin(Plugin):
    name = "my-plugin"

    # Single event type - inferred from type hint
    @hook
    def on_model_call(self, event: BeforeModelCallEvent):
        print(event)

    # Union types - registers for multiple events
    @hook
    def on_any_model_event(self, event: BeforeModelCallEvent | AfterModelCallEvent):
        print(event)
``

[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strands/hooks/_type_inference.py	91.42%	3 Missing ⚠️
src/strands/plugins/plugin.py	93.75%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[mkmeral]

nit: would it also make sense to export tool from here? I'm thinking the devx of:

from strands.plugins import Plugin, hook, tool

class MyPlugin(Plugin):
   ...

I don't have a strong preference here though

[JackYPCOnline]

Question: do we have validation check else where that hanldes edge cases like cls or self only method