test: Add edge case coverage for OpenAI mutate functionality and usage handling

[alxkm]

Enhanced test coverage for OpenAI module edge cases and validation

Strengthens test suites for OpenAiChatModelMutateTests and OpenAiUsageTests with additional validation scenarios:

OpenAiChatModelMutateTests improvements:

• Tests mutation with complex multi-value headers and custom authorization headers
• Validates handling of empty options and headers during mutation operations
• Ensures clone and mutate operations produce independent instances
• Tests builder validation and unchanged mutation scenarios

OpenAiUsageTests improvements:

• Validates proper handling of zero token counts across all usage metrics
• Ensures null token count values are safely converted to zero defaults

These additions improve reliability of the mutation pattern, ensure proper header handling for custom endpoints, and validate robust usage metric processing with edge case values.

[sobychacko]

Merged via dffec8d