fixed MCPClient to reuse session across tool calls

[omChauhanDev]

Please describe the changes in your PR. If it is addressing an issue, please reference that as well.

Fixes #3950

Issue : MCPClient was opening a new connection & re-initializing the MCP session on every single tool call. This caused unnecessary latency and repeated handshakes.

Solution : This PR makes MCPClient maintain a persistent session using AsyncExitStack, reused across all tool calls & schema requests until the client is closed.

MCPClient now requires an explicit lifecycle : either async with or start()/close().

Changes :

• Replaced 6 transport-specific methods with a single start()/close() lifecycle & unified _tool_wrapper
• Updated all MCP examples (39, 39a, 39b, 39c) to use async with

[codecov]

Codecov Report

❌ Patch coverage is 2.32558% with 42 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pipecat/services/mcp_service.py	2.32%	42 Missing ⚠️

Files with missing lines	Coverage Δ
src/pipecat/services/mcp_service.py	10.94% <2.32%> (+2.46%)	⬆️

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