fix(filter): preserve commas in exclusions

[Chemaclass]

Background

--exclude-filter 'test_a,{b}' treated the comma in the function name as a list separator. Combined with --filter test_a, it excluded both functions and reported No tests found instead of running test_a.

Changes

• Keep each repeatable --exclude-filter argument as one literal filter, including commas.
• Preserve the comma-separated format of BASHUNIT_EXCLUDE_FILTER configuration.
• Document the distinction and cover the reported Neotest selection scenario end to end.

Validation

• 83 focused tests and 102 assertions passed.
• ShellCheck and Bash syntax checks passed.
• make lint and git diff --check passed.

Checklist

• I updated the CHANGELOG.md to reflect the fix
• I updated the documentation to reflect the behavior

Closes #1340