Propagate MCP server startup and tool timeouts from ACP metadata

[stefandevo]

Purpose

This PR lets ACP clients tune Codex MCP startup and tool-call timeouts for per-session MCP servers. This is useful for blocking MCP tools that wait on user interaction and can legitimately run longer than Codex's default tool timeout.

Key Changes

• Reads startup_timeout_sec and tool_timeout_sec from each MCP server's _meta object.
• Forwards those values into McpServerConfig for HTTP and stdio MCP servers.
• Preserves existing behavior when timeout metadata is absent.
• Adds unit coverage for absent metadata, explicit metadata, and ACP JSON metadata deserialization.
• Documents the _meta timeout keys in both README files.

Checks

• cargo test
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• bash npm/testing/validate.sh
• node npm/testing/test-platform-detection.js