Add examples for google-ai-gemini module

[Bram--]

This adds examples for the Gemini Models: chat, streaming chat, embedding, token counting, function calling, structured responses, multi-modality, thinking mode, Gemini safety settings, and all batching permutations.

[dliubarskyi]

@Bram--

Error:  /home/runner/work/langchain4j-examples/langchain4j-examples/google-ai-gemini-examples/src/main/java/dev/langchain4j/example/gemini/Example12_BatchChatInline.java:[38,54] builder() is not public in dev.langchain4j.model.googleai.GoogleAiGeminiBatchChatModel; cannot be accessed from outside package
Error:  /home/runner/work/langchain4j-examples/langchain4j-examples/google-ai-gemini-examples/src/main/java/dev/langchain4j/example/gemini/Example14_BatchChatFromFile.java:[42,54] builder() is not public in dev.langchain4j.model.googleai.GoogleAiGeminiBatchChatModel; cannot be accessed from outside package

[Bram--]

@Bram--

Error:  /home/runner/work/langchain4j-examples/langchain4j-examples/google-ai-gemini-examples/src/main/java/dev/langchain4j/example/gemini/Example12_BatchChatInline.java:[38,54] builder() is not public in dev.langchain4j.model.googleai.GoogleAiGeminiBatchChatModel; cannot be accessed from outside package
Error:  /home/runner/work/langchain4j-examples/langchain4j-examples/google-ai-gemini-examples/src/main/java/dev/langchain4j/example/gemini/Example14_BatchChatFromFile.java:[42,54] builder() is not public in dev.langchain4j.model.googleai.GoogleAiGeminiBatchChatModel; cannot be accessed from outside package

@dliubarskyi Yes, this PR is dependent on: langchain4j/langchain4j#4156 and langchain4j/langchain4j#4175 and a version bump I suppose. I'll keep the PR as a draft for now