[MCP] describe_entities tool fixes and refactoring (#2900)

## Why make this change?

### Closes on
- #2827

Added fixes and refactored the describe_entities tool to support entity
metadata discovery for AI agents and LLM clients before performing CRUD
operations.

## What is this change?

- Expose entity descriptions with proper null handling
- Expose field description for entities of type stored-procedure with
parameter metadata
- Expose permissions for all entities with distinct values
- Added nameOnly parameter to return lightweight response (entity names
and descriptions only)
- Added entities parameter to filter results by specific entity names
- Implemented proper error handling using McpResponseBuilder pattern
- Added support for relationships, mappings, and primary key information
- Improved response structure with metadata section including mode and
count

## How was this tested?

Functional testing using Insomnia client by running DAB in localhost and
local SQL DB database

- MCP endpoint: http://localhost:5000/mcp
- JSON payload (details below)
- Validating the response

Expected (In-progress) format of entity description in response-

```
{
  "entities": [
    {
      "name": "entity-name",
      "description": "entity description",
      "type": "Table",
      "fields": [
        {
          "name": "field-name",
          "type": "unknown",
          "description": "field description"
        }
      ],
      "permissions": [
        "CREATE",
        "READ",
        "UPDATE",
        "DELETE"
      ],
      "primaryKey": ["id"],
      "relationships": [...],
      "mappings": {...}
    }
  ],
  "count": 1,
  "mode": "full"
}
```

## Sample Request(s)

```
POST http://localhost:5000/mcp

{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "name": "describe_entities"
  },
  "id": 1
}
```

```
POST http://localhost:5000/mcp

{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "name": "describe_entities",
    "arguments": {
      "nameOnly": true
    }
  },
  "id": 2
}
```

```
POST http://localhost:5000/mcp

{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "name": "describe_entities",
    "arguments": {
      "entities": ["Book", "Publisher"]
    }
  },
  "id": 1
}
```

---------

Co-authored-by: Anusha Kolan <[email protected]>

Author: souvikghosh04