Follow-up: extract valid-arms constant for DVL / IVL (dime T3)

[ottobolyos]

Origin

Dime Ultrareview cycle-1 on PR #241 (fix/device-validation-level-hardening), tracked finding T3 — surfaced by the simplification agent.

Finding

PR #241 collapses the two duplicated `Enum.IsDefined` setter throw blocks on `DeviceValidationLevel` and `InputValidationLevel` into a shared `ThrowIfUndefined` helper (finding L4), but the throw message text still hard-codes the valid-arms list twice:

• `"DeviceValidationLevel must be one of Ignore (0), Warning (1), Remove (2), Strict (3)."`
• `"InputValidationLevel must be one of Ignore (0), Warning (1), Remove (2), Strict (3)."`

The list `Ignore (0), Warning (1), Remove (2), Strict (3)` is a shared vocabulary (both enums share ordinals 0..3 by contract — that's what makes the load-time mirror in `AgentConfiguration.Normalize` safe). Extracting it to a shared string constant would keep the two setters (and any future third axis) DRY.

Rationale for defer

• The existing exception-shape pins (see `DeviceValidationLevelMigrationTests.DeviceValidationLevel_Setter_Exception_Carries_ParamName_And_ActualValue` and its `InputValidationLevel` sibling) assert `Message` contains the enum name — extracting the valid-arms list into a constant is safe as long as the enum name prefix stays. The tests would need review to confirm the assertion shapes survive.
• The refactor is quality-only, not a correctness fix. Bundling it with the H1/H2/M1/M2/M4 fixes in PR fix(common): tighten DeviceValidationLevel setter and preserve cycle-2 hardening #241 would blur the review target.

Suggested design (for the follow-up PR)

Add a shared constant near `AgentConfiguration.DeviceValidationLevelDefault`:

```csharp
private const string ValidationLevelValidArmsSuffix =
" must be one of Ignore (0), Warning (1), Remove (2), Strict (3).";
```

Then the setter calls become:

```csharp
ThrowIfUndefined(value, nameof(DeviceValidationLevel) + ValidationLevelValidArmsSuffix);
ThrowIfUndefined(value, nameof(InputValidationLevel) + ValidationLevelValidArmsSuffix);
```

Refs: dime cycle-1 T3 (simplification) — see PR #241's L4 refactor.