fix(oss): remove redundant ellipsis from Field() examples

[pawel-twardziak]

Remove redundant ... from Field(..., description="...") in models.mdx structured output examples, aligning with Pydantic v2 style already used in structured-output.mdx. Also adds a <Note> in the Pydantic tab explaining required vs optional fields, and an inline sentence in the TypedDict tab explaining the ... sentinel in Annotated syntax.

Type of change

Type: Fix typo/bug/link/formatting

Related issues/PRs

• GitHub issue:

Fixes #3098

Checklist

• I have read the contributing guidelines, including the language policy
• I have tested my changes locally using docs dev
• All code examples have been tested and work correctly
• I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed

Additional notes

Field(description="...") without ... is equivalent in Pydantic v2 fields with no default remain required. LangChain >= 0.3 requires Pydantic v2, so no backwards-compatibility concern.

[Sangram Mohanty (sangram11)]

Hi pawel-twardziak:
I verified the new change introduced in documents. I see that simply passing description in Field function makes it a mandatory field to be returned by LLM.

Although I tried to set it to optional it still returns me that field.
working_with_agents.ipynb

[Mason Daugherty (mdrxy)]

Thanks for verifying Sangram Mohanty (@sangram11)! You're right — in Pydantic v2, Field(description="...") without the ... sentinel is equivalent; fields with no default are required by default. So this change is purely cosmetic/idiomatic.

Re: optional fields still being returned — that's expected LLM behavior. Most models will populate all fields present in the schema regardless of whether they're marked Optional. The Optional typing affects Pydantic validation (allowing None), not the model's generation behavior.