ci: return UT to single runner

[XuPeng-SH]

What changed

Disable the opt-in four-way race-UT matrix introduced by #27815 and restore the MatrixOne workflow caller to its previous single-runner configuration:

• remove ut_sharded: true;
• restore ut_parallel: 8 in the caller.

Resulting CI behavior

CI #438 remains backward compatible and defaults ut_sharded to false. Therefore this caller now expands to exactly one all matrix entry:

• one UT Test on Ubuntu/x86 runner executes the complete race suite;
• UT_SHARD=all retains every light, HNSW, issues, embedded, heavy, engine, and plan stage;
• the shard summary job is skipped before runner allocation;
• the established required-check name remains unchanged.

This intentionally does not fully revert #27815. Its runner-local improvements remain: deterministic HNSW coverage with less duplicate work, global-mock restoration, temporary-file cleanup, package partition validation, and safer package grouping. Only the resource-expensive four-runner activation is rolled back.

CI #438 does not need to be reverted: it is opt-in, defaults to the old one-runner behavior, and its immutable-head checkout and fail-closed sharding support are harmless while disabled.

Validation

• the MatrixOne entrypoint is byte-for-byte identical to the parent of test: reduce race UT critical path #27815;
• YAML syntax parse: pass;
• git diff --check: pass;
• final delivery diff: one workflow file, 1 insertion and 4 deletions.

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