GH-3657: Fix DeepSeek tool call content null issue

[Acacian]

✨ Summary

This PR resolves the issue where DeepSeek chat completions with tool calls return an assistant message with content = null, leading to schema validation errors or downstream failures (e.g., in sglang or FastAPI with pydantic).

To address this, we explicitly set the assistant message content to "__tool_call__" when tool calls are present but content is null.

---

🛠 Changes

• In DeepSeekChatModel, updated buildGeneration(...) logic:
  • If content == null && toolCalls != empty, set content = "__tool_call__" to maintain compatibility.
• This change preserves downstream contract expectations, especially for clients expecting content to be a non-null string.
• No changes to the base AssistantMessage#getText() logic, keeping it backward compatible for other models.

---

💡 Context

DeepSeek sometimes returns:

{
  "role": "assistant",
  "content": null,
  "tool_calls": [...]
}

This results in downstream consumers (e.g., sglang, FastAPI with pydantic) throwing errors:

ChatCompletionMessageGenericParam.content Field required [type=missing]

By ensuring a placeholder content ("__tool_call__"), we:

• Avoid null pointer or validation errors
• Make the message displayable in logs or UIs
• Align behavior with other models that always return non-null content

---

🧪 Tests & Verification

• ✅ Added unit test: ChatClientResponseTests.whenAssistantMessageHasOnlyToolCalls_thenContentIsToolCallMarker
• ✅ Validated getText() returns "__tool_call__" when expected
• ✅ Verified compatibility with Generation.getOutput().getText()
• ✅ All existing tests passed (./mvnw clean verify)
• ✅ Checkstyle passed (-Ddisable.checks=false)
• ✅ Apache license headers added
• ✅ DCO signed
• ✅ Linear commit history (git log --oneline --graph confirmed)
• ✅ Rebased on latest main

---

📘 Documentation

• JavaDoc and inline comments added where applicable to clarify the fallback behavior.
• No changes required to public APIs or developer-facing contracts.

---

🔗 Closes #3657

Please review and let me know if you’d like any refinements.
Happy to iterate!

[dev-jonghoonpark]

Please update the copyright end year from 2024 to 2025.

[Acacian]

@dev-jonghoonpark
Thanks for the reminder regarding the import formatting and copyright year — I've made the changes accordingly.
I'll be more mindful of the style guidelines going forward. Appreciate your review!