* feat: enhance StdioTransport to prevent environment variable leakage
- Replaced os.environ.copy() with get_default_environment() to ensure only allowed environment variables are passed to the MCP server.
- Added tests to verify that ambient environment variables do not leak and that user-supplied environment variables can override defaults.
* feat: add environment variable filtering hook to StdioTransport
- Introduced an optional `_env_filter_hook` to allow extensions to modify the environment variables passed to MCP servers, enabling features like credential stripping.
- Updated tests to ensure the filtering hook is applied correctly after merging user-supplied and default environment variables.
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