fix: set default value for cache store

[oxSaturn]

Relates to:

Risks

Low.

Background

Seems we introduced this new CACHE_STORE recently, but users with old .env could be missing it, and the error is kind of unclear:

throw new Error(
                `Invalid cache store: ${cacheStore} or required configuration missing.`
            );

What does this PR do?

Set a default value for cacheStore so we won't break the agents with old .env file.

What kind of change is this?

Documentation changes needed?

Testing

Where should a reviewer start?

Detailed testing steps

[mcp97]

LGTM