Enable customization of table borders in ~/.myclirc

[rolandwalker]

Description

Styling table borders with a Pygments style turns out to have been already implemented in cli_helpers, per

• https://github.com/dbcli/cli_helpers/blob/9fb9f656ea8f4ab8230c2e4633526791d2d7438e/cli_helpers/tabular_output/tabulate_adapter.py#L163

In the mycli project, we just need to define Token.Output.TableSeparator and make it configurable by the user in ~/.myclirc.

xref #1663, #1654, #1653

Example with dimmed table borders:

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

1. Low: Missing regression test for the new output.table-separator mapping
   • Change is wired in code at mycli/clistyle.py:39 and exposed in defaults at mycli/myclirc:253, test/myclirc:251, but there’s no active test validating behavior.
   • Existing style tests are skipped (test/test_clistyle.py:12, test/test_clistyle.py:24), so a typo/regression in this mapping would likely pass CI.
   • Action: add a non-skipped unit test for style_factory_output() asserting {"output.table-separator": "<style>"} maps to Token.Output.TableSeparator in the returned style.

No correctness or security issues were evident in this PR diff beyond the test gap above.