Modify __init__ for sse_client when using thirdparty import

[ChenJiahaoST]

Add sse_client import in top init.py file.

Motivation and Context

When using mcp by lazy import in some Agent dev tools, cannot using sse_client as easily as stdio_client (mcp.client.sse.sse_client V.S. mcp.stdio_client), which is very uncomfortable and sometime may cause importing error.
After adding this change, user can import sse_client like stdio_client.

How Has This Been Tested?

I have tested this using a open source framework called LazyLLM, after adding this change, bug wiil be fixed.

Breaking Changes

No need to update, user can also use
from mcp import sse_client, stdio_client
rather than

from mcp import stdio_client
from mcp.client.sse import sse_client

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context