Refactor SQL agent

[ahuang11]

The goal of this PR is to simplify SQLAgent and make it run faster by reducing the number of steps to output SQL.

Changes:

1. Merged select_table, require_joins, find_joins into find_tables.
2. Changed the separator because <-> was really hard to look at, compared to :::
3. Reduced duplication in Planner (was showing double of the same schemas)
4. make_tables_model also highlights issues when joining

[philippjfr]

I think this is a nice improvement in principle but I've also found issues and am not confident I can track them all down.

[ahuang11]

Maybe test more and merge after announcement/release then?

[codecov]

Codecov Report

Attention: Patch coverage is 10.52632% with 68 lines in your changes missing coverage. Please review.

Project coverage is 57.65%. Comparing base (2b3c195) to head (6ff33a7).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lumen/ai/agents.py	7.40%	50 Missing ⚠️
lumen/ai/coordinator.py	0.00%	16 Missing ⚠️
lumen/ai/models.py	80.00%	1 Missing ⚠️
lumen/ai/utils.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1010      +/-   ##
==========================================
+ Coverage   57.47%   57.65%   +0.18%     
==========================================
  Files         109      109              
  Lines       14262    14212      -50     
==========================================
- Hits         8197     8194       -3     
+ Misses       6065     6018      -47     

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[ahuang11]