use FastAPI's natively injected background_tasks instance instead of creating a new one

[shahar4499]

Just a minor fix and a better practice. No functionality change.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
fastapi_mcp/server.py	66.66%	1 Missing :warning:

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