[Manohar Paturi (ManoharPaturi)]

Motivation & Context

The base64 decode path sizes its unpack format as len(raw) // 4, so a payload whose decoded length is not a multiple of 4 fails inside struct.unpack with a bare buffer-size error. Nothing in the message says which input's embedding was malformed, and the error type is an internal implementation detail of the decoder.

Description & Review Guide

• What are the major changes? The decode now checks alignment first and raises a ValueError naming the input index, the decoded byte count, and what was expected.
• What is the impact of these changes? Well formed payloads behave exactly as before. A malformed payload now surfaces a descriptive client-facing error instead of struct.error.
• What do you want reviewers to focus on? The test mirrors the existing base64 decoding test and feeds one well formed float plus a stray byte.

Related Issue

Fixes #9511

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