Don't create model automatically for agent

[ea-rus]

After decoupling agent from langchain handler mindsdb langchain model is not required for agent.
In this PR automatic model creation is removed.

Agent can be used as:

agent=connection.agents.create('my_agent', skills=['my_skill'], prompt_template='Answer the user input in a helpful way')

# or with default prompt template
agent=connection.agents.create('my_agent', skills=['my_skill'])

dependent on mindsdb/mindsdb#9525

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
mindsdb_sdk
agents.py	209	52	75%	33, 100, 103, 111, 119, 127, 135, 145, 148, 152, 154, 156, 158, 160, 162, 221, 234, 238–253, 265, 274–278, 285–286, 323, 335–336, 377, 425–426, 429, 436, 457–459, 463–467
databases.py	45	2	96%	109, 137
handlers.py	39	1	97%	77
jobs.py	97	7	93%	41, 53, 81, 85, 147–150
knowledge_bases.py	126	15	88%	58–61, 151, 155–162, 167–171, 250–253, 322, 339
ml_engines.py	42	3	93%	94, 126, 128
models.py	210	19	91%	109, 140–141, 222, 231, 233, 303, 339, 348, 372, 397, 491, 499, 518, 534, 542, 567, 571, 584
projects.py	63	1	98%	160
query.py	13	1	92%	14
skills.py	53	3	94%	43, 45, 49
tables.py	130	15	88%	140–142, 145, 165, 192, 203–204, 209, 224, 227, 321, 342–347, 356, 376
views.py	37	2	95%	105, 138
mindsdb_sdk/connectors
rest_api.py	224	42	81%	19–29, 35–36, 51, 54–55, 75–77, 98, 101, 108–111, 144–152, 173–174, 209–212, 226–227, 281–286
mindsdb_sdk/utils
mind.py	37	37	0%	1–102
openai.py	85	30	65%	37–40, 83–85, 107, 148–158, 215–216, 234–240, 258–276
table_schema.py	21	21	0%	1–54
TOTAL	1519	251	83%

Tests	Skipped	Failures	Errors	Time
23	0 💤	0 ❌	0 🔥	9.633s ⏱️

[tmichaeldb]

This default template would be better as a named constant

[ea-rus]

done

[ea-rus]

I don't merge this PR because we can't release sdk before mindsdb part is released