fix: remove extraneous source.description schema property; clarify entity.description covers MCP

[Copilot]

Why make this change?

entities.*.source.description existed in the JSON schema with documentation claiming it drove MCP tool discovery — but it was never wired to any C# model (EntitySource has no Description field) and was silently ignored at runtime. Meanwhile, entities.*.description — the field that actually surfaces in MCP — made no mention of MCP. Users following the schema would configure the wrong field and get no MCP output with no error.

What is this change?

• Removed entities.*.source.description from schemas/dab.draft.schema.json. The property never had a backing field in EntitySource and was dead schema surface area.
• Updated the description of entities.*.description to explicitly mention MCP:

- "Optional description for the entity. Will be surfaced in generated API documentation and GraphQL schema as comments."
+ "Optional description for the entity. Will be surfaced in MCP tool discovery, generated API documentation, and GraphQL schema as comments."

The correct config pattern for MCP-visible descriptions is:

"Todo": {
  "description": "Manages to-do items — surfaced in MCP, REST docs, and GraphQL schema.",
  "source": {
    "object": "dbo.Todos",
    "type": "table"
  }
}

How was this tested?

• Integration Tests
• Unit Tests

Schema-only change; no runtime behavior altered. The removed property had no backing model field and was never deserialized.

Sample Request(s)

N/A — schema documentation fix only.