[squad-plan] Fix error message compliance: validation_helpers.go, utc_offset.go, templatables.go, safe_outputs_validation.go, model_costs_pri [Content truncated due to length] #52171

Objective

Bring the error messages in these 5 files up to the repo's error-message style guide.

Context

Issue #52163 asks to repair error messages in batches of 5 files max. The lint-error-messages workflow (see run https://github.com/github/gh-aw/actions/runs/31510578209/job/93862987189) flagged these files as low-compliance:

• pkg/workflow/validation_helpers.go: 1/5 compliant (20%)
• pkg/workflow/utc_offset.go: 2/6 compliant (33%)
• pkg/workflow/templatables.go: 8/12 compliant (66%)
• pkg/workflow/safe_outputs_validation.go: 1/5 compliant (20%)
• pkg/workflow/model_costs_pricing_validation.go: 0/4 compliant (0%)

Implementation Guidance

1. Read .github/skills/error-messages/SKILL.md for the style guide before editing.
2. For each fmt.Errorf / NewValidationError call flagged non-compliant, rewrite the message using the template: [what's wrong]. [what's expected]. [example of correct usage].
3. Avoid standalone negative wording (invalid, cannot, must, failed) without pairing it with expected behavior and a concrete fix.
4. Use NewValidationError(field, value, reason, suggestion) for *_validation.go files where applicable; use fmt.Errorf with %w wrapping for operational errors, adding recovery guidance.
5. Keep changes surgical — do not alter error-handling logic or control flow, only message text.

Acceptance Criteria

• Running the lint-error-messages check locally (or via gh aw tooling used by that workflow) shows improved/100% compliance for these 5 files.
• make fmt run after changes.
• make test-unit (or targeted package tests for pkg/workflow) passes.
• No behavior change — only error message text updated.

Dependency Notes

Independent of other batches; can be worked in parallel with the other 7 sub-issues from this plan.

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