feat: detect AnthropicVertex provider as gcp.vertex_ai

[shreyas70]

Summary

• Detect AnthropicVertex / AsyncAnthropicVertex clients in Anthropic instrumentation and set gen_ai.provider.name to gcp.vertex_ai instead of anthropic
• Populate gcp.project and gcp.location span attributes from the Vertex client when available
• Add unit tests for provider resolution, GCP attribute enrichment, and end-to-end AnthropicVertex span coverage (including LangChain ChatAnthropicVertex path)

Test plan

• pytest test/instrumentation/anthropic/test_anthropic.py -k "vertex or resolve_provider or apply_vertex"
• Full test/instrumentation/anthropic/ suite passes in CI

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