refactor(agent-charm): move methods to charm utilities

[rene-oromtz]

Description

While trying to add charm code to the coverage report, noticed that it was very low on charm code side so before including coverage though on bumping up a little:

Current unreported coverage:

Name                                                            Stmts   Miss Branch BrPart  Cover
-------------------------------------------------------------------------------------------------
charms/testflinger-agent-host-charm/src/charm.py                  204    104     38      6    48%
charms/testflinger-agent-host-charm/src/common.py                  34      8      4      0    74%
charms/testflinger-agent-host-charm/src/config.py                  19      0      2      0   100%
charms/testflinger-agent-host-charm/src/defaults.py                 6      0      0      0   100%
charms/testflinger-agent-host-charm/src/supervisord.py             86     18     26      3    81%
charms/testflinger-agent-host-charm/src/testflinger_client.py      50      6      6      0    89%
charms/testflinger-agent-host-charm/src/testflinger_source.py      23      1      4      1    93%
-------------------------------------------------------------------------------------------------
TOTAL                                                             422    137     80     10    67%

Trying to keep it simpler for easier review so this PR moves some methods that can belong to a new module called "charm_utils", this covers all operation that belong to the charm lifecycle but that are not necessarily needed on charm code.

Also added a pydantic additional validation, instead of manually comparing config_dir and config_repo on each Charm Hook, I'm letting load_config() block the charm unit.

With the moved logic, coverage improved ~7%:

Name                                                            Stmts   Miss Branch BrPart  Cover
-------------------------------------------------------------------------------------------------
charms/testflinger-agent-host-charm/src/charm.py                  178     77     32      5    55%
charms/testflinger-agent-host-charm/src/charm_utils.py             30      0      4      0   100% <- New module
charms/testflinger-agent-host-charm/src/common.py                  34      8      4      0    74%
charms/testflinger-agent-host-charm/src/config.py                  25      0      4      0   100%
charms/testflinger-agent-host-charm/src/defaults.py                 6      0      0      0   100%
charms/testflinger-agent-host-charm/src/supervisord.py             86     18     26      3    81%
charms/testflinger-agent-host-charm/src/testflinger_client.py      50      6      6      0    89%
charms/testflinger-agent-host-charm/src/testflinger_source.py      23      1      4      1    93%
-------------------------------------------------------------------------------------------------
TOTAL                                                             432    110     80      9    74%

A follow up PR will add this coverage report

Resolved issues

N/A

Documentation

Web service API changes

Tests

Modify current tests to catch UncaughtCharmError triggered by Pydantic validation.
Also added unit tests for 100% coverage in charm_utils.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.85%. Comparing base (9cbb26f) to head (4559108).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #935   +/-   ##
=======================================
  Coverage   73.85%   73.85%           
=======================================
  Files         108      108           
  Lines       10313    10313           
  Branches      886      886           
=======================================
  Hits         7617     7617           
  Misses       2508     2508           
  Partials      188      188           

Flag	Coverage Δ		*Carryforward flag
agent	74.40% <ø> (ø)
cli	89.56% <ø> (ø)		Carriedforward from b2be3ff
device	59.86% <ø> (ø)		Carriedforward from b2be3ff
server	87.85% <ø> (ø)		Carriedforward from b2be3ff

*This pull request uses carry forward flags. Click here to find out more.

Components	Coverage Δ
Agent	74.40% <ø> (ø)
CLI	89.56% <ø> (ø)
Common	∅ <ø> (∅)
Device Connectors	59.86% <ø> (ø)
Server	87.85% <ø> (ø)

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[ajzobro]

Looks good, my few comments are mostly comment verbiage.

[rene-oromtz]

@ajzobro Thanks for the feedback, I added the suggested modifications, in case anything else is needed please let me know!

[ajzobro]

Looks good, thanks for considering my changes!