fix(provider): promote DeepSeek schema compatibility

[LeXwDeX]

Summary

• normalize DeepSeek tool schemas so the root is emitted as JSON Schema type object
• preserve the full workflow discriminated union and nested fields
• add a DeepSeek V4 Pro OpenAI-compatible regression test

Validation

• dev Typecheck: passed
• dev Unit Tests + generated artifacts + HttpAPI exerciser: passed
• dev E2E linux/windows: passed
• CodeQL: passed

Dev validation run: https://github.com/LeXwDeX/OpenCode-GraphAgent/actions/runs/31774660766