Rename methods and tweak API

[ahuang11]

1. Renames answer -> step_through
2. Renames Agent().invoke -> respond
3. Improve typing (messages: list)
4. Use only step.stream in step_through, and interface.stream in respond
5. Wrap output from step_through in AgentStepResults parameterized
6. Migrate agent chain tuples into AgentChainLink parameterized
7. Make LLM generate a short title for steps (I was confused why the LLM was doing consecutive SQLAgent calls) and widen the default min_width
8. Prevent plan step from flashing (due to empty string replace=True)

[codecov]

Codecov Report

Attention: Patch coverage is 33.02752% with 73 lines in your changes missing coverage. Please review.

Project coverage is 61.18%. Comparing base (049a997) to head (ff9a545).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lumen/ai/assistant.py	22.00%	39 Missing ⚠️
lumen/ai/agents.py	35.84%	34 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #738      +/-   ##
==========================================
- Coverage   61.22%   61.18%   -0.04%     
==========================================
  Files         101      101              
  Lines       12352    12371      +19     
==========================================
+ Hits         7562     7569       +7     
- Misses       4790     4802      +12     

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[philippjfr]

Looking good, but thinking about it again I'm not sure I love step_through and I'm also wondering if we actually need both methods or should simplify it to one method with an argument like render_output/render_final.

[ahuang11]

AgentStepResults did seem superfluous besides making the return type the same so I like that idea of merging those two methods

[ahuang11]

Maybe render_output can be controlled by the LLM too, e.g. prompt

Show me the table and the plot of the data

[ahuang11]

Okay, having the LLM decide on whether to render intermediate output works.

[ahuang11]

Table + Chat example