bug(cache): clear JSON cache artifacts on reset

[aviralgarg05]

Summary

Fix semantic cache clear behavior to remove JSON cache artifacts as well as legacy pickle files.

Why

Cache save/load favors JSON, but clear logic currently targets only one path pattern.

Tasks

• Update clear flow in nexum_ai/optimizer.py to remove active cache format files consistently
• Handle .json, .json.backup, and legacy .pkl safely
• Add/adjust tests in nexum_ai/tests/

Acceptance Criteria

• Clearing cache removes persisted cache artifacts
• No exceptions when files are already absent
• Tests validate file cleanup behavior

[coderabbitai]

🔗 Related PRs

#55 - feat: Implement semantic cache persistence to disk (#47) [merged]
#56 - Add EXPLAIN query plan command with AI optimization insights [merged]
#131 - feat: add EXPLAIN query plan command [merged]

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[aviralgarg05]

🤖 Claude Coder picking this up. (Timeout: 60min)

Using AI/LLMs to code? wont be assigned, try making genuine contributions