test: Isolate theme state in theme tests - #4039
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Some theme tests register and enable temporary themes globally. When pytest-xdist schedules those tests before example roundtrip tests in the same worker, the active custom theme can leak into later chart serialization and cause schema validation failures. Restore the default theme in cleanup blocks and unregister temporary themes after each mutating test. Suppress missing-theme errors during cleanup so cleanup does not mask the original test failure or fail after tests that intentionally unregister a theme.
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Some theme tests register and enable temporary themes globally. When pytest-xdist schedules those tests before example roundtrip tests in the same worker, the active custom theme can leak into later chart serialization and cause schema validation failures, as in https://github.com/vega/altair/actions/runs/27004820890/job/79693717966.
This PR restores the default theme in cleanup blocks and unregisters temporary themes after each mutating test. It also suppresses missing-theme errors during cleanup so cleanup does not mask the original test failure or fail after tests that intentionally unregister a theme.