fix(deepseek): reset tool_choice handling to prevent infinite loop when returnDirect=false (#4617)

[kuntal1461]

Summary
Fixes #4617 — When DeepSeekChatModel is forced to use a tool_choice with returnDirect=false,
the model repeatedly re-enters tool-call, causing an infinite loop.
This change resets tool_choice to auto after tool execution so the model can produce normal assistant text.

Changes

• Reset tool_choice to auto after tool execution in both internalCall and internalStream.
• Add DeepSeekChatModelToolChoiceResetTests to verify forced tool_choice behavior.

How to Reproduce

1. Define a tool with returnDirect=false
2. Force tool_choice to that tool
3. Observe repeated tool invocation (without this fix)

Expected Behavior
Tool is invoked once; model returns normal text.

Environment

• Observed on Spring AI 1.0.3
• Fix applied against main (1.1.0-SNAPSHOT)

Tests
mvn -pl models/spring-ai-deepseek -Dtest=org.springframework.ai.deepseek.DeepSeekChatModelToolChoiceResetTests test

DCO Sign-off
Signed-off-by: Kuntal Maity [email protected]