fix(cli): restrict stdio MCP environment inheritance

[d3f4w2]

Description

Stdio MCP servers currently receive every variable from process.env, including unrelated cloud credentials, database secrets, and API tokens. This change stops copying the ambient environment into each server configuration and lets the MCP SDK apply its platform-safe default environment, while preserving variables explicitly configured for that server.

This also avoids mutating serverConfig.env while constructing the transport.

Fixes #13010.

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

Not applicable; this changes the stdio MCP subprocess environment and has no UI surface.

Tests

• Added a regression test that places a sentinel secret in process.env, initializes an stdio MCP server with one explicit variable, and verifies only the explicit variable is passed to StdioClientTransport.
• npm test -- --run src/services/MCPService.test.ts — 21 passed
• targeted Prettier and ESLint checks passed
• CLI bundle build passed
• git diff --check passed

The standalone CLI tsc --noEmit command still reports existing NodeNext import-extension errors in core/ files outside this diff; the production esbuild bundle completes successfully.