Update SLO aware router with Admit Request Plugin

[kaushikmitr]

This pull request introduces admission control logic to the SLOAwareRouter and improves robustness by adding nil request checks to lifecycle hooks. It also refactors the scoring and prediction logic for better separation of concerns and testability, and adds comprehensive unit tests for the new admission controller behavior.

Admission control and validation:

• Implements the AdmitRequest method for SLOAwareRouter, enabling admission control based on SLO predictions. The method checks if a request has a valid pod according to SLO context and rejects requests otherwise. It also handles nil requests and missing SLO contexts gracefully.
• Registers SLOAwareRouter as an AdmissionPlugin by updating the interface assertions.
• Adds extensive unit tests for AdmitRequest, including edge cases (nil request, missing SLO context, valid/invalid pod scenarios, concurrent access, and error messages).

Robustness improvements:

• Adds nil request checks with debug logging to all request lifecycle hooks (PreRequest, ResponseReceived, ResponseStreaming, ResponseComplete) to prevent panics and improve traceability.
  Prediction and scoring refactoring:

• Refactors the generatePredictions method to remove the unused state parameter and simplifies prefix cache score retrieval by using the SLO context instead of recalculating.

• Moves prefix cache score calculation into the Score method before prediction generation, ensuring scores are available in the SLO context for all candidate pods.

• Updates the call to generatePredictions in Score to match the new signature, removing the state parameter.

• Fixes logic for determining if all pods are invalid by considering SLOs

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