Support Google Agent Platform (Vertex AI) as a BYOK provider

[gael-ft]

I saw the #711 on this topic but:

fixed/answered/superseded #711 (comment)

Well Agent Platform (a.k.a. Vertex AI) is still not supported ...

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Re-opening it as it is impacting us at the moment and we'd like to avoid switching to other tools that already supports it (e.g. Claude)

[rotationtv1-crypto]

Commit reviewed