Preserve reasoning_content on assistant tool-call message in DeepSeek…

[i-zoufeng]

… streaming

When deepseek-reasoner is used with streaming and tool calls, the API requires the assistant message replayed in subsequent rounds to include reasoning_content; otherwise it returns "thinking is enabled but reasoning_content is missing in assistant tool call message".

Spring AI dropped the field on three independent paths:

1. DeepSeekStreamFunctionCallingHelper#merge did not accumulate the reasoning_content delta nor forward the prefix flag across chunks.

2. DeepSeekChatModel#createRequest passed null for reasoning_content when re-serializing a DeepSeekAssistantMessage. Even after reading the typed field, Prompt#mutate() (called from buildRequestPrompt) downgrades AssistantMessage subclasses to the plain superclass via instructionsCopy, so subclass-only fields cannot survive a round trip; only metadata does.

3. DeepSeekApi#chatCompletionStream emits each pre-tool-call chunk (reasoning delta, content delta) as its own one-element window for streaming UX and only merges chunks once the tool-call window opens. The chunk that triggers ToolCallingManager#executeToolCalls therefore carries only tool_calls, and the assistant message stored in the next round's conversation history has empty content and no reasoning_content.

Fixes:

• Merge reasoning_content and forward prefix in the chunk-window merger.
• Mirror reasoningContent and prefix from DeepSeekAssistantMessage into the message metadata, and read them back from metadata in createRequest, so the round trip works whether the message is a DeepSeekAssistantMessage or a plain AssistantMessage created by Prompt#instructionsCopy.
• In DeepSeekChatModel#internalStream, maintain per-stream-call accumulators for content and reasoning_content. Before invoking executeToolCalls, enrich the assistant message with the accumulated values so the next round carries the full pre-tool-call context.

Adds regression tests covering chunk-merge accumulation, the AssistantMessage to ChatCompletionMessage round trip, and the streaming-path enrichment.

Fixes gh-5898

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