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 responses now include token usage details when provided, improving visibility into API consumption metrics (input, output, and total tokens).

[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: ef9ccd99-3a44-4b98-91aa-e8f0136ba879

📥 Commits

Reviewing files that changed from the base of the PR and between dcc7b7c and bf9a0a6.

📒 Files selected for processing (1)

• .changeset/gemini-image-usage-metadata.md

✅ Files skipped from review due to trivial changes (1)

• .changeset/gemini-image-usage-metadata.md

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📝 Walkthrough

Walkthrough

The changeset documents that @tanstack/ai-gemini now reports Gemini image token usage from usageMetadata instead of leaving usage undefined.

Changes

Gemini image usage release note

Layer / File(s)	Summary
Patch changeset .changeset/gemini-image-usage-metadata.md	Records the Gemini image adapter update that maps usageMetadata into inputTokens, outputTokens, and totalTokens.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Possibly related PRs

• TanStack/ai#655: Modifies the same Gemini image adapter path to populate ImageGenerationResult.usage from response.usageMetadata.

Suggested reviewers:AlemTuzlak

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the Gemini image usageMetadata parsing fix.
Description check	✅ Passed	The description explains the fix and test plan, and is mostly complete even though the template checklist and release-impact sections are missing.
Linked Issues check	✅ Passed	The changes implement the #330 requirement by mapping Gemini usageMetadata into ImageGenerationResult.usage.
Out of Scope Changes check	✅ Passed	The PR stays within scope; it only updates the Gemini image adapter and adds the matching changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[AlemTuzlak]

@tecoad hey man, thank you for this but it's already been fixed on main!