docs: Update customer_support_small_model example to use non-deprecated methods and add type annotations

[jet-isnt-haha]

Description

Updated the customer_support_small_model.ipynb example to use non-deprecated API methods and improve TypeScript type safety.

Changes

1. Replace deprecated _getType() method

• Before: if (trimmedHistory.at(-1)._getType() === "ai")
• After: if (trimmedHistory[trimmedHistory.length - 1].getType() === "ai")
• The _getType() method signature is deprecated and should be replaced with getType()
• Also replaced .at(-1) with [trimmedHistory.length - 1] to fix TypeScript "possibly undefined" error (ts(2532))

2. Add missing type annotation

• Location: addConditionalEdges callback in billing_support node
• Added: async (state: typeof StateAnnotation.State) => type annotation
• Ensures consistency with other conditional edge definitions in the example

Type of Change

• Documentation update
• Bug fix (non-breaking change which fixes TypeScript errors)

Testing

• Verified code changes compile without TypeScript errors
• Confirmed deprecated method warnings are resolved

Motivation

This PR addresses two issues in the example notebook:

1. API Deprecation: The _getType() method is deprecated and should be replaced with the public getType() API
2. TypeScript Safety: Adding explicit type annotations and using safer array access patterns improves type safety and prevents potential runtime errors

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