fix: set default to none for optional field without default value

[vincent0426]

Motivation and Context

Closes #1402

pydantic treats Optional type as required if there is no default value

import json
from pydantic import BaseModel, Field
from typing import Optional


class TestModel(BaseModel):
    param_1: str | None = Field(description="Optional without default")
    param_2: Optional[str] = Field(description="Optional without default")

schema = TestModel.model_json_schema()
print("Pydantic schema:")
print(json.dumps(schema, indent=2))

output

Pydantic schema:
{
  "properties": {
    "param_1": {
      "anyOf": [
        {
          "type": "string"
        },
        {
          "type": "null"
        }
      ],
      "description": "Optional without default",
      "title": "Param 1"
    },
    "param_2": {
      "anyOf": [
        {
          "type": "string"
        },
        {
          "type": "null"
        }
      ],
      "description": "Optional without default",
      "title": "Param 2"
    }
  },
  "required": [
    "param_1",
    "param_2"
  ],
  "title": "TestModel",
  "type": "object"
}                

How Has This Been Tested?

Unit tested on 5 scenarios

• required
• optional with type | none
• optional with Optional[type]

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[felixweinberger]

PR attempting to resolve the same issue: #1408

[felixweinberger]

Between this and #1408 if it turns out we actually need this change and not a fix to our Pydantic usage, I'd probably prefer this one for greater simplicity.

Will figure out whether this is the right approach though - there may be ways we can better use Pydantic tools rather than work around them here.

[Kludex]

I don't think we should add a default value if the type accepts None.

[vincent0426]

I think pydantic treats Optional[str] as required, do we not want to treat it as Optional if specify

https://github.com/pydantic/pydantic/blob/main/docs/migration.md#snippet_4:~:text=f2%3A%20str%20%3D%20%27abc%27-,Required%2C%20can%20be%20None,-f3%3A%20Optional%5Bstr

[Kludex]

I think pydantic treats Optional[str] as required

Yes, which is correct.

---

Optional keyword doesn't mean optional, it means nullable in Python.