Prevent concurrent mutation in Google GenAI dimension lookup

[dangzitou]

Summary

• Fixes GoogleGenAiTextEmbeddingModel.dimensions() mutates a shared static HashMap, throwing ConcurrentModificationException under concurrent use #6763.
• GoogleGenAiTextEmbeddingModel.dimensions() no longer mutates the shared static lookup map when model is unknown.
• Unknown models now use the existing AbstractEmbeddingModel fallback path.

Issue

Closes #6763

Change

The current computeIfAbsent call on a shared mutable HashMap can throw ConcurrentModificationException under concurrent access when multiple unknown model names are requested simultaneously. This change removes side-effectful map writes from the hot path by querying dimensions once and only assigning via the existing fallback path.

Why

• Applies first-principles reasoning on concurrency: mutable shared state should not be modified without proper synchronization in read-mostly paths.
• Preserves behavior for all known-model names and removes rare concurrency crash class.

Validation

• Static concurrency and control-flow review of dimensions() branching.
• Manually reasoned equivalent outputs for supported models.
• mvn test / full project CI not run in this environment.