[Test-Proxy] Support minimal regex flags

We're always multiline when matching, but we need to be able to enable `ignore case` at the very least.

- [ ] Modify `RecordedTestSanitizer` basic implementation to honor new optional argument `flags`
- [ ] Modify `Admin/AddSanitizer` code to honor new property, if necessary.

Start by locating the basic RecordedTestSanitizer implementation and the Admin/AddSanitizer code mentioned in the checklist. Trace how sanitizer arguments and properties are handled, then verify that the optional flags value enables at least ignore-case matching while preserving the existing multiline behavior.

Written by the indexing model from the issue text.