fix(core): sanitize interrupted tool call arguments

[Alexxigang]

Summary

• sanitize interrupted streaming tool-call argument payloads before they are persisted into memory
• add a shared JSON normalization helper used by the accumulator and provider converters
• add regression tests for accumulator recovery plus DashScope/OpenAI formatter fallback behavior

Why this fix

When a streaming tool call is interrupted mid-JSON, AgentScope can persist a partial ToolUseBlock.content string into memory. On the next request, the DashScope/OpenAI formatter path reuses that invalid JSON as tool-call arguments, which can trigger provider-side 400 errors.

This patch fixes the bug at two layers:

• ToolCallsAccumulator now normalizes incomplete raw argument fragments to either serialized structured input or {} before the message is finalized
• DashScopeToolsHelper and OpenAIMessageConverter now defensively sanitize historical ToolUseBlock.content before sending it back to providers

Fixes #1147.

Validation

• mvn -pl :agentscope-core -am spotless:apply
• mvn -pl :agentscope-core -am "-Dtest=ToolCallsAccumulatorTest,DashScopeToolsHelperComprehensiveTest,OpenAIMessageConverterTest,ToolCallJsonUtilsTest" test