Fix list_accessible_customers to return JSON-formatted string

[PhilipJohnBasile]

Fixes issue where FastMCP converts list return type into multiple text items that get concatenated.

Changes

• Changed return type from List[str] to str
• Returns JSON object with customer_ids array and count
• Prevents FastMCP from concatenating customer IDs into single string
• Claude now receives parseable JSON instead of concatenated IDs

Before

123456789023456789013456789012

After

{
  "customer_ids": ["1234567890", "2345678901", "3456789012"],
  "count": 3
}

[PhilipJohnBasile]

@DeanLukies can you take a look at this?

[Raibaz]

I am honestly puzzled by this, as FastMCPs renders List[str] as one string per row, with a newline between each string.

Do you have more context on where you experienced the issue of strings being concatenated?

[jayflaherty75]

What is the impact of this change? It seems like it breaks contract to solve a problem elsewhere (FastMCP). Typically, I'd add a JSON version of the function returning the result from the original. In some dark corners, a JSON flag would be added but we don't talk about that. =b