feat: modify env interface with Symbol

[sunrabbit123]

This pull request introduces a significant refactor to the handling of environment variables and improves testing for MCP (Model Context Protocol) connections and server interactions. The main changes include replacing the env and envMapper properties with a unified externalEnv property, adding support for required and optional environment variables, and enhancing test coverage with mock implementations and additional assertions.

Refactor of Environment Variable Handling:

• Replaced env and envMapper with externalEnv in connectMcp and related functions to streamline environment variable management (src/cli.ts, src/index.ts, src/mcp.ts). [1] [2] [3] [4] [5]
• Introduced RequiredEnv and OptionalEnv symbols to differentiate between mandatory and optional environment variables in StdioMcpConnection (src/mcp.ts).
• Updated extractEnvFromName to handle required and optional environment variables, logging warnings for missing required variables (src/mcp.ts).

Test Enhancements:

• Added mocks for StdioClientTransport and improved test coverage for connectMcp, including handling of required/optional environment variables and multiple MCP servers (src/mcp.test.ts). [1] [2] [3]
• Enhanced createServer tests to include cleanup of resources (e.g., closing clients and servers) and added tests for tool listing and tool call requests (src/server.test.ts). [1] [2] [3]