[FLINK-40513][runtime] Introduce multimodal AI functions for image understanding

[haruki-830]

This PR adds multimodal AI function support to Flink CDC Pipeline transforms.

AI_IMAGE_COMPLETE uses the standard OpenAI-compatible vision chat protocol. AI_IMAGE_EMBED and SupportsImageEmbedding are added as extension points, but image embedding is not yet supported by any built-in client. DashScope-specific embedding is intentionally out of scope.

Brief change log

• Added SupportsImageTextGeneration and SupportsImageEmbedding.
• Added AI_IMAGE_COMPLETE and AI_IMAGE_EMBED.
• Added runtime parsing, type inference, capability validation, and UDF precedence handling.
• Implemented OpenAI-compatible vision chat using MIME detection and Base64 data URLs.
• Added tests for supported image formats, Base64 encoding, null input, and invalid input.
• Updated the English and Chinese documentation.

Verifying this change

This change added tests and can be verified as follows:

• Updated AiFunctionsTest and AiFunctionParserTest.
• Updated OpenAiCompatibleModelClientTest and OpenAiCompatibleModelClientFactoryTest.
• Updated FlinkPipelineAiFunctionITCase.
• Verified the tests with both the default Flink profile and -Pflink2.
• Verified the complete repository with mvn spotless:check.

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? docs and JavaDocs

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Was generative AI tooling used to co-author this PR?

• Yes (OpenAI Codex)

Generated-by: OpenAI Codex

[yuxiqian]

Thanks Haruki for the nice work, just left some minor comments.