Fix sparse provider message block arrays

[qwertyquest]

Bug

Ran into an issue causing an invalid type issue with OpenAI Responses API. Neuron's provider message mappers can emit sparse PHP arrays for message content blocks.

When a message contains mixed blocks like [ReasoningContent, TextContent], the mapper drops unsupported blocks with array_filter(array_map(...)). array_filter() preserves numeric keys, so if index 0 is removed and index 1 remains, PHP JSON-encodes the result as an object instead of a list.

That breaks OpenAI Responses with errors like:

Invalid type for 'input[n].content': expected one of an array of objects or string, but got an object instead.

Direct example:

• Assistant message content blocks: [reasoning, text]
• Mapped PHP array becomes: [1 => ['type' => 'output_text', 'text' => 'Hello']]
• JSON becomes:

{"1":{"type":"output_text","text":"Hello"}}

instead of:

[{"type":"output_text","text":"Hello"}]

Fix

Wrapped provider mapBlocks() methods with array_values to reindex after filtering and added regression tests.