fix: support multimodal requests

[shenald-dev]

Modified isValidMessage in src/index.js to allow msg.content to be an array, supporting OpenAI multimodal requests where the content payload contains multiple objects instead of a plain string. Also updated tests/api.test.js to add an integration test for multimodal API endpoint requests and added a unit test for the isValidMessage helper itself covering the multimodal format.

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PR created automatically by Jules for task 15844512300851733743 started by @shenald-dev

[google-labs-jules]

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[shenald-dev]

Looks good, merging this in. CI is green and everything checks out 👍

Add support for multimodal message content as array in isValidMessage and related tests.

[google-labs-jules]

Looks good, merging this in. CI is green and everything checks out 👍

Add support for multimodal message content as array in isValidMessage and related tests.

Acknowledged.