Draft changelog: PPU credit balance endpoint

[mintlify]

Summary

Adds a changelog entry announcing the new GET /2/usage/credits endpoint for checking Pay-per-use credit balance.

Changes

• Added an Aug 24, 2026 entry to changelog.mdx under X API v2 covering the new endpoint and what it means for PPU users.

Context

Follows #329, which added the GET /2/usage/credits endpoint to the Usage docs.

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Project	Status	Preview	Updated (UTC)
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