docs(alloydb): Add AlloyDB Vector Search Benchmark Colab

[vinay0410]

Description

Fixes #

Checklist

Testing

• I have tested this change on a live environment and verified it works as intended.

Compliance & Style

• I have followed the Sample Guidelines.
• The README is updated with all relevant information (if applicable).
• If this PR adds a new sample directory: I updated the CODEOWNERS file.

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Post-Approval Actions

• Please merge this PR for me once it is approved

[gemini-code-assist]

Code Review

This pull request adds an end-to-end test test_alloydb_vector_search_benchmark to run a benchmark notebook with reduced parameters in CI. The reviewer identified that the notebook filename and variable keys for the cluster and instance names are incorrect, which would cause the test to fail, and provided a code suggestion to correct them.

[gemini-code-assist]

The notebook filename and the variable keys for the cluster and instance names do not match the actual notebook file (columnar_engine_accelerated_hnsw_vector_search_benchmark.ipynb). This will cause the end-to-end test to fail as the file won't be found, and the variables won't be correctly substituted.

Please update the notebook filename and use cluster_name and instance_name as the keys in the variables dictionary.

Suggested change

	"alloydb_vector_search_benchmark.ipynb",
	variables={
	"project_id": project_id,
	"region": region,
	"cluster_id": cluster_name,
	"instance_id": instance_name,
	"columnar_engine_accelerated_hnsw_vector_search_benchmark.ipynb",
	variables={
	"project_id": project_id,
	"region": region,
	"cluster_name": cluster_name,
	"instance_name": instance_name,