H-3922: Add Google Vertex AI calls to shared AI request logic

[CiaranMn]

🌟 What is the purpose of this PR?

We have a function that provides a standard interface for calling AI models, and translates it into the specific format each provider requires.

This PR adds support for Google Vertex AI calls to that function. This means (1) it can be more easily used from anywhere in future, (2) requests are logged in the same standard format, and (3) usage is properly tracked.

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing

📜 Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

🛡 What tests cover this?

• None yet.

❓ How to test this?

1. Only possible locally, via uploading a document file (which triggers Gemini analysis).

📹 Demo

TODO

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 298 lines in your changes missing coverage. Please review.

Project coverage is 21.03%. Comparing base (70aa0bb) to head (484c3dd).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
.../shared/get-llm-response/get-google-ai-response.ts	0.00%	51 Missing ⚠️
...et-google-ai-response/rewrite-schema-for-google.ts	0.00%	51 Missing ⚠️
...nse/get-google-ai-response/google-cloud-storage.ts	0.00%	39 Missing ⚠️
...ai-response/map-google-messages-to-llm-messages.ts	0.00%	31 Missing ⚠️
...activities/shared/use-file-system-file-from-url.ts	0.00%	30 Missing ⚠️
...t-google-ai-response/map-parts-and-upload-files.ts	0.00%	18 Missing ⚠️
...-activities/infer-metadata-from-document-action.ts	0.00%	16 Missing ⚠️
...ta-from-document-action/get-llm-analysis-of-doc.ts	0.00%	14 Missing ⚠️
...shared/get-llm-response/google-vertex-ai-client.ts	0.00%	14 Missing ⚠️
.../shared/get-llm-response/get-anthropic-response.ts	0.00%	6 Missing ⚠️
... and 9 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6290      +/-   ##
==========================================
- Coverage   21.21%   21.03%   -0.18%     
==========================================
  Files         573      580       +7     
  Lines       19664    19827     +163     
  Branches     2872     2928      +56     
==========================================
  Hits         4171     4171              
- Misses      15441    15604     +163     
  Partials       52       52              

Flag	Coverage Δ
apps.hash-ai-worker-ts	1.34% <0.00%> (-0.05%)	⬇️

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