test: Add comprehensive test coverage for ModelObservationContext and ModelUsageMetricsGenerator

[alxkm]

This PR adds comprehensive unit test coverage for ModelObservationContext and ModelUsageMetricsGenerator classes, focusing on validation, edge cases, and metrics generation scenarios.

Changes Made

ModelObservationContext Tests:

• Added validation tests for empty and whitespace-only operation types and providers
• Added tests for different AI provider types (Ollama, OpenAI, Anthropic)
• Added generic type testing with complex object types (Integer, Boolean)
• Added getter/setter behavior validation including multiple response updates
• Added edge cases for empty string requests and responses

ModelUsageMetricsGenerator Tests:

• Added test for zero token usage metrics generation
• Added test for null prompt/generation tokens with valid total tokens
• Added validation of metric registry behavior and counter values
• Added testing of different token type tags

Test Coverage Improvements

Validation: Empty/null/whitespace parameter handling
Edge Cases: Zero values, null combinations, empty strings
Provider Support: Multiple AI provider validation
Metrics Generation: Token usage tracking and registry validation
Type Safety: Generic type parameter testing

[sobychacko]

Merged via c893629.