Lazy import sse_starlette to reduce import time

[CNSeniorious000]

Move from sse_starlette import EventSourceResponse to where EventSourceResponse is used to defer importing uvicorn.

Motivation and Context

Fixes #1770

How Has This Been Tested?

import sys, mcp

assert "uvicorn" not in sys.modules

Breaking Changes

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

[Copilot]

Pull request overview

This PR optimizes module import time by implementing lazy imports for sse_starlette.EventSourceResponse, which prevents the early loading of the uvicorn dependency when the mcp module is imported. The changes move the import statements from module level to function/method level, placing them right before EventSourceResponse is used.

Key Changes:

• Removed top-level from sse_starlette import EventSourceResponse imports
• Added function-level lazy imports in four locations where EventSourceResponse is used
• Maintains consistent pattern across both sse.py and streamable_http.py

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/mcp/server/sse.py	Moved EventSourceResponse import inside response_wrapper function within connect_sse method
src/mcp/server/streamable_http.py	Added three lazy imports for EventSourceResponse in _handle_post_request, _handle_get_request, and _replay_events methods

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