fix(ai-gemini): parse usageMetadata in image adapter response

[tecoad]

Summary

• Fix Gemini image adapter always returning usage: undefined by mapping usageMetadata from the API response to the ImageGenerationResult.usage field
• Maps promptTokenCount → inputTokens, candidatesTokenCount → outputTokens, totalTokenCount → totalTokens
• Consistent with how the OpenAI image adapter already handles usage data

Closes #330

Test plan

• Generate an image using Gemini adapter and verify usage is populated with token counts
• Verify that when usageMetadata is absent, usage gracefully falls back to undefined
• Run pnpm test:lib and pnpm test:types on the ai-gemini package

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Gemini image generation requests now properly include token usage information in responses, enabling better tracking and monitoring of API consumption metrics.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 233cad24-153b-4d33-b4d5-f214d566b805

📥 Commits

Reviewing files that changed from the base of the PR and between 228e0e3 and dcc7b7c.

📒 Files selected for processing (1)

• packages/typescript/ai-gemini/src/adapters/image.ts

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📝 Walkthrough

Walkthrough

The Gemini image adapter now parses usageMetadata from API responses and populates the usage field in ImageGenerationResult, mapping prompt, candidates, and total token counts accordingly.

Changes

Cohort / File(s)	Summary
Gemini Image Adapter packages/typescript/ai-gemini/src/adapters/image.ts	Added conditional parsing of usageMetadata from Gemini response to populate usage object with mapped token counts; returns undefined when metadata is absent.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 A rabbit hopped through Gemini's nest,
Found tokens hiding in the test,
With prompt and output counts aligned,
The usage field is now defined! 🌟

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: fixing the Gemini image adapter to parse usageMetadata in responses.
Description check	✅ Passed	The description covers the changes made and references the test plan, but doesn't explicitly mark the template checklist items as complete.
Linked Issues check	✅ Passed	The code changes directly address issue #330 by mapping Gemini's usageMetadata fields to ImageGenerationResult.usage with correct field transformations.
Out of Scope Changes check	✅ Passed	The changes are limited to the Gemini image adapter and directly address the linked issue without introducing unrelated modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

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