[Repo Assist] Fix docs: clarify why Tweet fields are inferred as optional in JsonProvider example

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Closes #1073

Problem

The JsonProvider documentation for the Twitter stream example contained a misleading explanation:

"the tweet.User property has been inferred as optional (meaning that a tweet might not have an author?)"

This suggests the optional inference is because tweets can lack a User, which is incorrect. It also doesn't explain why RetweetCount and Text are optional.

Fix

Update the paragraph to explain the real reason: TwitterStream.json contains both tweet objects and other event types (e.g. delete events) with completely different schemas. When the type provider merges heterogeneous samples, any field absent from some samples is inferred as optional.

The updated text:

"The reason is that TwitterStream.json contains not only tweet objects but also other event types (such as delete events) with a completely different schema. When the type provider merges multiple heterogeneous sample objects, any field that does not appear in all samples is inferred as optional."

Test Status

• Build passes (documentation-only change, no code changed)
• Tests pass (no code changed)

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