[Platform] Derive VertexAI API host from location for regional and data-residency endpoints

[rserry]

Q	A
Bug fix?	no
New feature?	yes
Docs?	yes
Issues	Fix #2283
License	MIT

The VertexAI bridge hardcoded aiplatform.googleapis.com for every request, so
regional and data-residency (jurisdictional) deployments could not be reached —
a blocker for anyone with data-residency requirements (e.g. EU healthcare/GDPR).

This derives the API host from the location already passed to the factory,
via a small RegionAwareTrait::resolveHost() helper shared by the Gemini and
Embeddings model clients:

• null / global → aiplatform.googleapis.com (unchanged)
• eu / us → aiplatform.eu.rep.googleapis.com / aiplatform.us.rep.googleapis.com
• any region, e.g. europe-west1 → europe-west1-aiplatform.googleapis.com

// Regional endpoint
Factory::createPlatform(location: 'europe-west1', projectId: 'my-project');

// EU data-residency endpoint
Factory::createPlatform(location: 'eu', projectId: 'my-project');

[chr-hertel]

Hi @rserry, thanks for raising this issue and jumping on providing a solution - pretty much appreciated!

I made some minor comments - i don't use Vertex myself, but also noted this change now with the examples and I think we should consider this a breaking change => meaning to add a note about it in UPGRADE.md

[chr-hertel]

Please move this section to docs/components/platform/vertexai.rst instead - we keep the README tidy, but the rst files are deployed to symfony.com docs

[chr-hertel]

let's keep it short:

Suggested change

	* Derive the API host from the configured `location` so regional (e.g. `europe-west1`) and data-residency (`eu`, `us`) endpoints are used; `null`/`global` keep the existing `aiplatform.googleapis.com` host
	* Derive the API host from the configured `location` to support regional and and data-residency endpoints