ci: refactor 14 E2E jobs into 3 matrix jobs (1094 → 521 lines)

Consolidate individual E2E workflow jobs into matrix strategy:
- 9 Python E2Es → 1 `e2e-python` matrix job
- 3 JS E2Es → 1 `e2e-js` matrix job
- 2 Java E2Es + new void-optimization → 1 `e2e-java` matrix job

Also absorbs standalone `e2e-java-void-optimization.yaml` into ci.yaml,
fixing its mutable `@v6` tag to `@v8.0.0` in the process.

Gate job now references 6 needs instead of 17.

Author: KRRT7