fix: handle literal string when env var is unset

[vchomakov]

Fix: Handle literal ${REDIS_URL} string when environment variable is unset

Problem

When the REDIS_URL environment variable is not set, gemini-cli passes the raw "${REDIS_URL}" string as the --url parameter instead of omitting it or passing an empty value. This causes the Redis MCP server to attempt parsing the literal string "${REDIS_URL}" as a Redis URI, which fails with a parsing error.

Root Cause

The issue occurs because:

1. gemini-cli constructs command line arguments that include --url ${REDIS_URL}
2. When REDIS_URL is unset, shell expansion doesn't occur, so the literal string "${REDIS_URL}" gets passed
3. The server tries to parse this as a valid Redis URI, which fails

Solution

Filter out the "${REDIS_URL}" placeholder string.
This allows the server to gracefully fall back to using individual Redis connection parameters (host, port, etc.) with their default values when no valid URL is provided.

Impact

• Fixes Redis MCP server startup failures when installed from PyPI and used with gemini-cli
• Maintains backward compatibility with existing configurations
• No breaking changes to the API or configuration options

[tishun]

This is just. Sad.

Thanks for fixing it!