fix(openapi): preserve explicit null values in OpenAPI examples

[sbawabe]

Description

Refs: Request from Sarah Bawabe

Fixes a bug where explicit null values in OpenAPI response examples were being incorrectly replaced with schema example values. For example, balance_max: null in a tiered rate structure (representing an unbounded upper tier) was being replaced with "100000000" from the schema's example field.

Link to Devin run: https://app.devin.ai/sessions/dcba61f67aa34cc5aaac1eb92ee496a5
Requested by: Sarah Bawabe (@sbawabe)

Root Cause

The nullish coalescing operator (??) was treating explicit null values as nullish and falling back to schema examples. Changed to use explicit undefined checks to preserve null values in examples.

Changes Made

• ExampleConverter.ts: Changed property example resolution from propExampleFromParent ?? this.maybeResolveSchemaExample(property) to an explicit undefined check, so explicit null values are preserved
• ExampleTypeFactory.ts: Added early return for example === null in nullable schema handling, and changed property example resolution to use inExample check to respect explicit presence in the example object
• Added test fixture balance-max-null with an OpenAPI spec containing:
  • A RateTier schema where balance_max is nullable
  • An explicit response example with 3 tiers where the third tier has balance_max: null
• Added test case with assertions verifying tiers[2].balance_max is null (not replaced with "100000000")
• Updated valtown snapshots to reflect correct behavior (null values now preserved instead of replaced with schema examples)

Testing

• Unit tests added/updated
• All existing tests pass
• Lint checks pass
• All CI checks pass

Human Review Checklist

• Verify the valtown snapshot changes are expected behavior:
  • profileImageUrl: null instead of "profileImageUrl" (schema example)
  • lastInsertRowid omitted when not present in the example
• Verify the nullish coalescing → undefined check change doesn't break other example generation scenarios
• Confirm the example === null early return in ExampleTypeFactory is correct for nullable schemas
• Check that the fix correctly distinguishes between "property absent from example" vs "property explicitly set to null"

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