test(typescript): expand multimodal coverage

[Jonesxq]

Summary

Adds the requested comprehensive TypeScript SDK tests for multimodal helpers. The new suite exercises text construction, image and audio factories, real temporary-file round trips, mocked audio downloads and failures, format fallbacks, and MIME inference for images, audio, documents, and extensionless files.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Docs only
• Tests only
• CI / tooling
• Breaking change

Test plan

• cd sdk/typescript && npm test -- tests/multimodal_comprehensive.test.ts (16 passed)
• cd sdk/typescript && npm test -- tests/multimodal.test.ts tests/multimodal_comprehensive.test.ts --coverage --coverage.reportsDirectory=coverage-407 (22 passed)
• cd sdk/typescript && npm run lint
• cd sdk/typescript && npm test (822/823 passed; the existing resource-sensitive memory leak test reports 31.4 MB growth against a 25 MB threshold and also fails in isolation)

Test coverage

Using the unchanged multimodal.test.ts plus the new comprehensive suite, src/ai/multimodal.ts line/statement coverage increases from 38.14% to 86.59%; branch coverage is 89.58%.

• I ran tests for the surface(s) I changed locally.
• New code paths are covered by tests in this PR (no bare additions).
• No code or coverage baselines were removed or lowered.
• The coverage gate check is green in CI before requesting review.

Checklist

• I have read docs/CONTRIBUTING.md and docs/DEVELOPMENT.md.
• The commit follows conventional-commits style.
• The commit is signed (no signing configuration is available in this checkout; recent main commits are also unsigned).
• I have linked the related issue.

Related issues / PRs

Closes #407

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.