Proposed Changes

Fix Rack HTTP status code deprecation warnings by updating deprecated symbols to their modern equivalents:

• Replace :payload_too_large with :content_too_large (HTTP 413)
• Replace :unprocessable_entity with :unprocessable_content (HTTP 422)

Motivation: Rack PR #2137 deprecated these symbols. This change eliminates deprecation warnings when running tests and ensures compatibility with future Rack versions.

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