fix(mcp): prevent agent hang by checking session closure state

[Ratish1]

Description

This PR resolves a bug where MCPClient would attempt to schedule tool calls to a background event loop that had already finished its main task but whose thread was still alive. It prevents indefinite hangs when 4xx/5xx errors cause the background loop to exit before the thread is joined.

• Updated _is_session_active to check if `_close_future`` is done.

Related Issues

Fixes #1334

Documentation PR

N/A

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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