feat(firebase-ai): add inlineData support to Developer API

[ryanwilson]

Description

The Developer API was missing support for inlineData parts, which was already implemented in the Vertex AI API.

This change copies the inlineData parsing logic from the Vertex AI parsePart function to the _parsePart function in the Developer API.

The existing test file, developer_api_test.dart, has been updated to include a test case for parsing inlineData parts, ensuring the functionality is working as expected.

Tested in the iOS simulator with a manual change to the sample (not included, since it likely warrants further example changes)

Related Issues

Fixes #17449
There was a previous attempt at this but it was closed: #17451

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Breaking Change

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