fix(usage): default token counts to 0 when provider returns nil

[Rogermax]

Summary

• Adds default: 0 to input_tokens, output_tokens, and total_tokens in the Usage struct

Why fix here instead of the monolith?

The error TypeError: Parameter 'input_tokens': Can't set Ai::Usage.input_tokens to nil occurs when:

1. The AI provider returns a response without usage information
2. TypeCoerce tries to construct the Usage struct with nil values
3. Sorbet's runtime type checking throws TypeError because the fields are required Integers

The monolith code already has error handling for Ai::Error, but TypeError is a different exception class that bypasses this handling.

Options considered:

1. ❌ Catch TypeError in the monolith - This is a workaround, not a fix. Every consumer would need to add this handling.
2. ❌ Make fields T.nilable(Integer) - Forces all consumers to handle nil values.
3. ✅ Add default: 0 - If the provider doesn't return usage data, we default to 0 (unknown usage). This is the most graceful solution.

Root cause investigation

The Usage struct was introduced in commit bd8a706 (Sep 23, 2025) without defaults. The issue likely started appearing after commit 02e83fd which "dropped backfilling from Mastra" - Mastra may have previously been filling in these values, and now they can arrive as nil in edge cases (partial timeouts, incomplete responses, etc.).

Test plan

• All existing specs pass (83 examples, 0 failures)
• Verified the change is backwards compatible

Fixes FCT-45236

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