fix(embed): join startup workers and preserve service failures

[XuPeng-SH]

What type of PR is this?

• BUG
• Test and CI

Which issue(s) this PR fixes:

Related: #28646

What this PR does / why we need it:

Embedded cluster startup can launch CN workers concurrently, but the previous implementation stored heterogeneous startup errors in one atomic.Value. Different concrete error types can panic while being stored, and a synchronous LOG/TN failure could return while an already-launched CN worker was still starting. That lets rollback overlap startup and hides the original failure.

The startup path now owns one error slot per service, joins every launched CN worker before returning, keeps errors in service configuration order, preserves errors.Is/errors.As, and stops scheduling services after a synchronous infrastructure failure. The fix applies to initial and incremental startup. The regression tests use the existing startFn seam, so they do not start real services or add sleeps.

The error wrapper uses MatrixOne's typed internal error to satisfy the repository SCA contract. The concurrent-error oracle has been updated to assert the resulting diagnostic while retaining the cause and ordering checks.

Validation

• Focused startup selection is non-empty and passes in normal mode.
• TestDoStartLockedConcurrentErrors: -race -count=100, passed (1.192s).
• TestDoStartLockedJoinsCNBeforeReturningInfrastructureFailure: -race -count=100, passed (1.194s).
• Full pkg/embed normal: passed (49.268s).
• Full pkg/embed race: passed (55.684s).
• go vet ./pkg/embed/...: passed.
• Incremental golangci-lint --new-from-rev origin/main ./pkg/embed/...: 0 issues.
• git diff --check: passed.

The tests were run from the rebased candidate worktree at main ee0c2bf563bf27ff2c80891bb57a1de5886bf90b with a freshly built CPU CGo/native dependency set. The first vet attempt hit a full /tmp before analysis; it was rerun with disk-backed temporary storage and passed.

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