Replace AvailableServices usage for Vertex, Anthr, Docs clients

What does this MR do and why?

Replace a number remaining AvailableServices.find_by_name calls with unit_primitive_name.to_sym for:

• Vertex client,
• Anthropic client
• and DocsSearch client

These three call sites combined together because I worked on all three call sites (we were supplying UP name into Service object) and these call sites where well-tested manually, so I have a pretty high level of confidence replacing service object (build around UP) to the UP's title (symbol).

How to set up and validate locally

It should be no-op, but you can refer to related MRs where we migrated these call sites to the UP usage:

• Vertex Client: !202231 (merged)
• Anthropic Client: !201598 (merged)
• DocsClient: !201280 (closed) - it was not merged, but instructions are relevant.

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Related to #564979 (closed)