Cover toolbox dataZoom filter modes for stacked data

Copilot noted that the AxisProxy change touches weakFilter and raised concern about empty mode. Add focused coverage for both paths so the stacked-dimension filtering contract is explicit.

Constraint: toolbox dataZoom exposes filterMode and may inject value-axis filters before user zoom actions.

Rejected: Revert empty mode to raw axis dimensions | tests show stackResultDimension is the rendered value and should be emptied directly.

Confidence: high

Scope-risk: narrow

Directive: Keep weakFilter and empty coverage when changing stacked dataZoom dimensions.

Tested: ./node_modules/.bin/jest --config test/ut/jest.config.cjs --coverage=false test/ut/spec/component/dataZoom --runInBand

Tested: npm run checktype

Tested: ./node_modules/.bin/eslint test/ut/spec/component/dataZoom/AxisProxy.test.ts

Tested: git diff --check -- test/ut/spec/component/dataZoom/AxisProxy.test.ts

Not-tested: npm run checkheader not rerun for this test-only follow-up; previous run failed on pre-existing unrelated files.