Accept empty content array - transform it to null

[mzegla]

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[Copilot]

Pull request overview

This PR updates OVMS LLM chat input handling to accept OpenAI-style content: [] message payloads by preserving them during request parsing and normalizing them downstream to null, avoiding upfront request rejection and keeping the processing pipeline consistent across LM/VLM chat paths.

Changes:

• Accept empty content arrays in OpenAI chat parsing and defer normalization to a new EmptyContentNormalizationProcessor.
• Adjust TextContentNormalizationProcessor to flatten only text-only content arrays (leave mixed-modality arrays intact for image decoding) and run it for both LM and VLM chat paths.
• Update and extend unit/integration tests to reflect the new parsing + normalization behavior.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/test/llm/input_processing/text_content_normalization_processor_test.cpp	Updates expectations: mixed-modality content arrays are no longer flattened.
src/test/llm/input_processing/input_processing_integration_test.cpp	Verifies text-only array flattening on both LM and VLM paths.
src/test/llm/input_processing/empty_content_normalization_processor_test.cpp	Adds unit tests for empty-array-to-null normalization behavior.
src/test/http_openai_handler_test.cpp	Updates parsing test: empty content arrays are now accepted and preserved.
src/llm/io_processing/input_processors/text_content_normalization_processor.hpp	Updates processor documentation to match new behavior and scope.
src/llm/io_processing/input_processors/text_content_normalization_processor.cpp	Ensures only text-only arrays are flattened; mixed arrays are left for downstream image handling.
src/llm/io_processing/input_processors/empty_content_normalization_processor.hpp	Introduces new processor to normalize empty content arrays to null.
src/llm/io_processing/input_processors/empty_content_normalization_processor.cpp	Implements empty-array-to-null normalization for ChatHistory inputs.
src/llm/io_processing/input_processor.cpp	Inserts new processor into the chat input processing chain and adjusts processor ordering.
src/llm/BUILD	Adds the new processor sources/headers to the LLM input processors Bazel target.
src/llm/apis/openai_completions.cpp	Stops rejecting empty content arrays during OpenAI chat message parsing.