Upgrade tenacity to v9 to enable compatibility with Neo4j GraphRAG

[krcm0209]

Is your feature request related to a problem? Please describe.
Neo4j GraphRAG for Python requires tenacity v9 or higher. However, google-adk currently locks tenacity to less than v9 in its dependencies. This prevents dependency resolution when using Neo4j GraphRAG alongside google-adk, making it impossible to install both packages together.

Describe the solution you'd like
Update the tenacity dependency in google-adk to allow version 9 or higher. This would resolve the compatibility issue and enable projects to use Neo4j GraphRAG with google-adk without dependency conflicts.

Describe alternatives you've considered

• Forking google-adk to change the dependency constraint (not ideal for maintainability).
• Avoiding Neo4j GraphRAG (undesirable, as this is a required dependency).

Additional context
Related packages:

• neo4j/neo4j-graphrag-python (requires tenacity >=9)
• jd/tenacity (tenacity v9 is latest major release)

This upgrade unblocks compatibility for projects relying on both google-adk and Neo4j GraphRAG.

[marcoskogaita]

Adding to the discussion, the library changes for >=9.0.0 where not breaking changes.
I don't know why the author changed the major version because there were no signature changes in the library, only bug fixing and typedef additions.

Also, the latest version is 7 months old and the library is supper stable

[speedstorm1]

@speedstorm1 - I can work on this