Fix DeepSeek interleaved tool-call streaming

[SmartAI]

Summary

• Buffer OpenAI-compatible streamed tool calls by index instead of rejecting a second call.
• Emit completed calls in index order for sequential execution without mixing arguments.
• Keep requesting single calls and retain validation for incomplete, changed, or duplicate identities.
• Add a local HTTP end-to-end regression that writes then reads a file using interleaved calls.

Validation

• Reproduced the original compatibility error before the fix.
• Isolated PR checkout: 160 tests passed, 1 skipped.
• Focused Ruff and mypy checks passed.
• No live DeepSeek API verification.

Behavior note

Tool-call events are emitted when the tool-call response finishes; text continues streaming as before.