[XuPeng-SH]

Change

Enable the existing four-way race-UT shard workflow for MatrixOne Linux CI (ut_sharded: true, keeping ut_parallel: 8) and update the workflow contract test. The shard map is checked for complete, disjoint package coverage; this change does not filter or skip tests.

Evidence

• GOWORK=off go test -mod=readonly -race -count=1 -timeout=180s -run '^(TestUTShardStagesFormCompleteDisjointMap|TestUTShardStagesRejectUnknownValues|TestValidateUTReportRejectsDuplicateTestRootsAcrossShards)$' ./optools passed.
• All 18 change-scope Node tests passed; targeted YAML parsing, actionlint, and git diff --check passed.
• Historical sharded run 34916576954 completed all four UT shards. Their runtimes were roughly 18m (light), 36m (issues), 28m (embedded), and 20m (heavy/plan); required checks finished in about 46–49m. This is feasibility evidence from an older tree, not a matched current-base speedup measurement.

Dependency

The historical run failed only at the final UT summary because the reusable workflow declared five unused EE_DB_* secrets required, while the control-only summary job does not select the ci environment. matrixorigin/CI#466 makes those inputs optional while keeping the S3 secrets required. That PR's workflow validation passed, but it is still awaiting review. This PR is Draft until #466 is merged and a current-head sharded CI run passes.

Current CI validation boundary

The standard pull_request_target run 39545739855 loads the trusted caller workflow from base main at 04fe057b, where ut_sharded is still false. It therefore validates the current source tree through the unsharded path; it does not exercise this PR's caller toggle. The historical sharded run above confirms the four-shard path on an older tree, but a current-base sharded run is still outstanding.