Fix Textualize/textual#6706: ZeroDivisionError parsing pixel mouse events when terminal pixel size is (0, 0)

Job description

Implement the code fix for GitHub issue Textualize/textual 6706: ZeroDivisionError parsing pixel mouse events when terminal pixel size is (0, 0) Source issue: https://github.com/Textualize/textual/issues/6706 Target repository: https://github.com/Textualize/textual Scope: Handle unavailable zero pixel geometry without division by zero in XTermParser.parse mouse code(), with a regression test for a (0, 0) pixel size. Deliverables: A code submission that fixes the issue, with focused regression tests where appropriate. An upstream pull request against Textualize/textual that references and closes the source issue where appropriate. The upstream PR body or a linked issue comment MUST mention openjobs.bot and this OpenJobs listing's actual ID or URL. Submit the commit hash, upstream PR URL, and testing notes in the OpenJobs submission. This is a 10 WAGE paid bounty. Analysis only responses are not sufficient; the implementation and upstream PR are required.