fix: tell type checkers that the config options are strings (trivial)

[tonyandrewmeyer]

Applicable spec: N/A

Overview

ops<=2.12 wrongly has self.config[x] typed as str, when actually it could be an int, float, bool, or str, depending on the config type. We're fixing this in ops:#1183, but that will break static checking that currently assumes that the config is a str (because ops doesn't validate the schema, so all options will be bool|int|float|str).

This PR adds a typing.cast call where the config values are loaded and used where the type should be str.

Rationale

Covered in the overview.

Juju Events Changes

N/A

Module Changes

N/A

Library Changes

N/A

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation is generated using src-docs
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex)

No docs changes required.

[yanksyoon]

/canonical/self-hosted-runners/run-workflows 22207de

[arturo-seijas]

/canonical/self-hosted-runners/run-workflows 2bc4cd7

[github-actions]

Test coverage for 2bc4cd7

Name                Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------
src/charm.py          540     35    184     33    91%   162, 203-206, 370, 429->433, 582, 613, 619, 636-640, 670, 705-706, 764, 773, 877->890, 889, 891, 896-897, 957, 977, 984, 1074, 1083, 1095, 1116, 1125, 1144, 1148, 1177, 1230, 1362, 1384, 1391->1393, 1436->exit, 1448, 1458, 1496
src/cos.py             21      0      0      0   100%
src/exceptions.py      17      1      2      1    89%   41
src/state.py           34      0      4      0   100%
src/types_.py          16      0      0      0   100%
---------------------------------------------------------------
TOTAL                 628     36    190     34    91%

Static code analysis report

Run started:2024-04-16 08:52:05.609996

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 4586
  Total lines skipped (#nosec): 1
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):