[BUG] Required values should have an option to be not nullable

[alexzerntev]

Description

I had this param:

    - name: value1
      type: number
      description: "value that can be extracted from another tool (required)"

The value1 is required. I need to forbid its calling with null value (so the LLM is obliged to first find this value from another tool), but it is not possible now.

Expected Behavior

The ability to state if the value is nullable e.g:

    - name: value1
      type: number
      description: "value that can be extracted from another tool (required)"
      nullable: false

Environment

• MXCP Version: 0.9.2
• Python Version: 3.13
• Operating System: MacOS
• Installation Method: uv

[bgaidioz]

The MXCP .yml schema doesn’t allow specifying whether a parameter can be null or not.
The ability to specify a default value doesn’t make a parameter nullable—it only allows the LLM to omit the argument entirely.

In early PR #18 we even made parameters nullable by default. At the time, I believed all arguments would eventually be passed to SQL, where null values are always acceptable, and since .yml files were documenting SQL parameters, it felt natural for them to be nullable as well.

JSON Schema (used by MCP) allows declaring a nullable parameter:

{ "type": ["integer", "null"] }

Our generaed pydantic models currently make both tools and prompt parameters nullable.

We need to add a nullable: flag to .yml files, or define a syntax that allows parameters to declare multiple types and include null among them (e.g., { "anyOf": ["string", "null"] }).
Such a syntax would also support type unions like string | date or int | list[int]. That would be backward incompatible.