[ADD] ai_mcp: add Odoo MCP server

Purpose:
To give users the ability to connect odoo to any MCP client (Gemini CLI, Claude Code, n8n, etc) and to make them able to define complex automations using LLMs.

Capabilities:
- Read: The MCP server provides search and read_group tools to allow MCP clients to read data and generate summaries/analytics.
- Write: The MCP server provides the write tool to allow MCP clients to create and update records in the database.

How to use
- The user has to generate an API key which is used to authenticate the user so that the MCP server can execute tools based on user permissions.
  - Login into Odoo with your username and password.
  - Click on the user image on the top right.
  - Go to My preferences and then select the Security tab.
  - Click on Add Api Key to generate the API key. Choose the scope as MCP.
- To connect to the MCP server, you will add the following to the MCP client config. Replace the database_url with the url of your Odoo database and api_key with the API key generated in step 1. The structure can change a bit based on the MCP client but you will always need the URL and headers.
```json
{
    "mcpServers": {
        "Odoo": {
          "type": "http",
          "url": "<database_url>/mcp",
          "headers": {
            "Authorization": "Bearer <api_key>"
          }
        }
    }
}
```

This commit introduces a dedicated bearer_mcp auth method on the MCP HTTP route that validates API keys strictly against scope='mcp', preventing RPC-scoped tokens from accessing the MCP server and vice versa.

This commit also adds an access check to _ai_tool_get_fields to make sure that the current user has access to the model before retrieving the fields.

Community PR: https://github.com/odoo/odoo/pull/268589
Upgrade PR: https://github.com/odoo/upgrade/pull/10432

task-6139938