Add code assistant tutorial feature

[Rajkumar7633]

• Created new code-assistant tutorial module
• Added CodeAssistant.java with code generation, review, explanation, and debugging tools
• Added comprehensive README with usage instructions
• Updated parent pom.xml to include new module
• All tests pass successfully

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[Rajkumar7633]

No existing issue. This PR adds:

1. New code assistant tutorial feature

2. Critical code quality improvements (unsafe Optional.get() fixes)
   Problem:

3. Missing code assistant tutorial for developers

4. Unsafe Optional.get() calls that could cause NoSuchElementException crashes

5. Inconsistent error handling in LLM flow processing
   Solution:

6. Added new code-assistant tutorial with 4 tools (generate, review, explain, debug code)

7. Replaced unsafe .get() calls with orElseThrow() for better error handling

8. Added meaningful error messages for debugging

9. Removed duplicate condition checks
   Testing Plan:

Unit Tests:

• All 24 modules pass successfully (BUILD SUCCESS)
• ./mvnw test completed in 2:23 min with no failures

Manual E2E Tests:

• Tested code assistant tutorial with GOOGLE_API_KEY
• Verified all 4 tools work correctly via web interface
• Tested existing tutorials still work after changes