feat: configure similarity top-K in the VertexAiRagRetrieval tool

[pgentile]

Link to Issue or Description of Change

Problem:

Add support for the similarity top-K parameter in the VertexAiRagRetrieval tool

Solution:

New similarityTopK constructor parameter in the constructor of the VertexAiRagRetrieval class.

The similarity top-K is then added to the LLM request, when the model is a Gemini one and when the GOOGLE_GENAI_USE_VERTEXAI env var is enabled. Otherwise, the similarity top-K parameter is transmitted to the retrieveContexts method.

Testing Plan

Updates of the VertexAiRagRetrievalTest unit test.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

[INFO] +-- com.google.adk.tools.retrieval.VertexAiRagRetrievalTest
[INFO] | +-- [OK] processLlmRequest_otherModel_doNotAddVertexRagStoreToConfig - 1.298 ss
[INFO] | +-- [OK] processLlmRequest_gemini2Model_addVertexRagStoreToConfig - 0.006 ss

...

[INFO] +-- com.google.adk.tools.retrieval.VertexAiRagRetrievalTest
[INFO] | +-- [OK] processLlmRequest_otherModel_doNotAddVertexRagStoreToConfig - 0.084 ss
[INFO] | +-- [OK] processLlmRequest_gemini2Model_addVertexRagStoreToConfig - 0.001 ss
[INFO] | +-- [OK] runAsync_withResults_returnsContexts - 0.190 ss
[INFO] | +-- [OK] runAsync_noResults_returnsNoResultFoundMessage - 0.003 ss
[INFO] | +-- [OK] processLlmRequest_gemini2Model_withoutSimilarityTopK_addVertexRagStoreToConfig - 0 ss

Manual End-to-End (E2E) Tests:

• Use the tool new parameter in a production agent.
• Check both code branches based on the GOOGLE_GENAI_USE_VERTEXAI env var.
• Count retrieved chunks from the RAG corpus. The number of chunks must match the configured similarity top-K

Checklist

• I have read the CONTRIBUTING.md document.
• My pull request contains a single commit.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

N/A