feat: O(1) function call lookups

[AmaadMartin]

PR Description

Optimized function call lookups in adk-python to be O(1) by introducing a cached function_call_index on the Session model.
This refactor also introduces Session.get_matching_function_call() natively to eliminate leaky abstractions into static helper functions, resulting in a cleaner and highly performant architecture.

Complexity & Performance

• Function Call Index lookups operate natively in O(1) time.
• Re-indexing is purely incremental, executing strictly over new events rather than recalculating the entire index.
• E2E benchmarks demonstrate O(1) scalability up to 10,000+ events per session efficiently.

Testing

• All 194 unit tests successfully passing with full line and branch coverage.
• Validated via isolated E2E testing framework manually triggering >10000 sequential events.

Review Board

• Passes Complexity Auditor review (Removed over-engineered injection logic).

[rohityan]

Hi @AmaadMartin , Thank you for your contribution! We appreciate you taking the time to submit this pull request.
Closing this PR here as it belongs to adk-python-community repo.
We highly recommend releasing the feature as a standalone package that we will then share through: https://google.github.io/adk-docs/integrations/