Various tweaks for assistant, agents and utilities

[philippjfr]

• Refactors the PlanningAssistant
• Minor tweaks for various agents
• Minor tweaks to make get_schema utility more robust

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 65 lines in your changes missing coverage. Please review.

Project coverage is 54.41%. Comparing base (17763d0) to head (f8120ea). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lumen/ai/assistant.py	0.00%	45 Missing :warning:
lumen/ai/utils.py	0.00%	13 Missing :warning:
lumen/ai/agents.py	0.00%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #728 +/- ## ========================================== - Coverage 54.52% 54.41% -0.12% ========================================== Files 99 99 Lines 12168 12193 +25 ========================================== Hits 6635 6635 - Misses 5533 5558 +25 ```

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