Keep agents moving in parallel MCP tool calls without session conflicts

Resolved a race condition in MCPTools where parallel tool calls using a header_provider would each independently spin up their own MCP session instead of sharing one, leaving the agent in a stuck state. Session creation is now correctly coordinated so that concurrent tool calls share a single session as intended.

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Details:

Fixes duplicate session creation when multiple MCP tool calls execute in parallel with header_provider configured
Eliminates the agent hang caused by conflicting concurrent sessions
No configuration changes required; the fix applies automatically to all MCPTools setups using header_provider