feat: add inventory response validator

[JustAzul]

Pull Request Description

Summary

This PR adds a new utility function to validate and process Steam inventory API responses by merging asset data with their corresponding descriptions.

What Changed

• Added: New file src/utils/validate-response.js containing the validateInventoryResponse function
• Purpose: Processes Steam inventory responses to combine asset data with detailed item descriptions

Functional Impact

The Steam inventory API returns data in two separate arrays:

• assets: Contains basic item information (classid, instanceid, etc.)
• descriptions: Contains detailed item metadata

This utility:

1. Validates the response data structure
2. Creates a lookup map indexed by classid and instanceid
3. Merges assets with their corresponding descriptions into unified objects
4. Returns an array of complete item objects ready for use

This simplifies downstream processing by providing a single, enriched dataset instead of requiring consumers to manually join assets with descriptions.

Technical Details

• Returns null for invalid/empty input
• Uses object key composition (classid_instanceid) for efficient lookups
• Employs Object.assign() to merge asset and description properties

[kodus-review]

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[JustAzul]

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