fix: use plain errors for clickhouse model messages

[immanuwell]

Tiny vet cleanup in the ClickHouse model package.

Go 1.26 flags fmt.Errorf(s) when s is already a built string. No formatting needed here, so errors.New(s) is the straight shot. same behavior, less noise.

Repro before:

go test ./pkg/model/clickhouse

It failed with:

non-constant format string in call to fmt.Errorf

After:

go test ./pkg/model/clickhouse

passes

[sunsingerus]

LGTM. Tiny, correct vet cleanup — fmt.Errorf(s) → errors.New(s) at 3 sites in pkg/model/clickhouse. Verified:

• Functional equivalence: identical *errors.errorString output when s has no % verbs; no callers use errors.Is/errors.As on these errors.
• Sweep is complete for the package: go vet ./pkg/model/clickhouse/ flags exactly these 3 sites.
• Go version claim is accurate: go.mod declares go 1.26.2.
• Format-injection rationale is real (SQL interpolates user-controlled cluster names), though theoretical in Go (memory-safe fmt).

Approved.