docs: explain MCP env expansion

[JSap0914]

Summary

• add a dedicated mcpServers path/environment expansion subsection
• document supported $VAR, ${VAR}, ${VAR:-fallback}, and Windows %VAR% syntax
• call out unsupported {{VAR}}, ${env:VAR}, and ~, plus missing variables expanding to an empty string
• mirror the warning in the MCP setup tutorial

Fixes #28228.

Verification

• npx prettier --check docs/reference/configuration.md docs/cli/tutorials/mcp-setup.md

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request improves the documentation for the Gemini CLI by clarifying how environment variables are expanded within mcpServers configurations. It provides users with clear guidelines on supported syntax, platform-specific behaviors, and common pitfalls, ensuring a more predictable configuration experience.

Highlights

• Documentation Update: Added a dedicated section explaining environment variable expansion for mcpServers configuration.
• Syntax Clarification: Documented supported formats including $VAR, ${VAR}, ${VAR:-fallback}, and %VAR% for Windows, while noting unsupported patterns like {{VAR}} and ~.
• Best Practices: Provided clear examples for cross-platform path configuration and warned about the behavior of missing variables.

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[github-actions]

📊 PR Size: size/S

• Lines changed: 35
• Additions: +35
• Deletions: -0
• Files changed: 2

[gemini-code-assist]

Code Review

This pull request updates the documentation to detail environment variable expansion support within mcpServers configuration settings, including syntax rules and usage examples. There are no review comments, and I have no feedback to provide.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

[gemini-cli]

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