fix: check if embedding is base64 encoded before convert it to float …

[Suzu008]

What this PR does

• 修复上游 perf(embedding): default embedding creation to base64 openai/openai-node#1312 导致的部分 embedding provider 返回数据被错误地按 base64 处理
• 在进行 embedding 的 base64 转换到 float array 的操作前判断 embedding数据类型是否为 string

Fixes #
#6142
#6470
#6976 ?

Why we need it and why it was done in this way

The following tradeoffs were made:

The following alternatives were considered:

Links to places where the discussion took place:

Breaking changes

If this PR introduces breaking changes, please describe the changes and the impact on users.

Special notes for your reviewer

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• PR: The PR description is expressive enough and will help future contributors
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• Documentation: A user-guide update was considered and is present (link) or not required. You want a user-guide update if it's a user facing feature.

Release note