[fix] Attempt to run the Firebase CLI via npx if it's not installed

[samedson]

Description

When you run the flutterfire_cli via the Gemini CLI, we're running into users that don't have the Firebase CLI installed. Because the Firebase MCP server runs via npx, users don't install the Firebase CLI to their PATH.

To ensure compatibility, we're suggesting that we run the Firebase CLI via npx as a backup if it doesn't exist.

Type of Change

• ✨ feat -- New feature (non-breaking change which adds functionality)
• 🛠️ fix -- Bug fix (non-breaking change which fixes an issue)
• ❌ ! -- Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 refactor -- Code refactor
• ✅ ci -- Build configuration change
• 📝 docs -- Documentation
• 🗑️ chore -- Chore

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