fix(mcp): scope stdio handshake timeout

[wgqqqqq]

Summary

Follow-up to #888.

• keep the 30s timeout for local stdio MCP initialize/handshake only
• leave regular local MCP requests, especially long-running tools/call, without the handshake timeout
• remove timed-out local requests from the pending map
• avoid spawning a login shell for every macOS child process by caching the merged PATH once and reusing existing platform PATH expansion

Verification

• cargo test -p bitfun-services-core
• cargo test -p bitfun-services-integrations --features mcp connection::tests -- --nocapture
• cargo check -p bitfun-core --features product-full
• git diff --check