Deprecate Constraint::CHECK_MODE_NONE (#896)

`Constraint::CHECK_MODE_NONE` (`0x00000000`) has been defined since
early versions but is never referenced anywhere in the codebase and has
no meaningful semantic—a bitmask of zero applied to any flag check is a
no-op.

## Changes

- **`Constraint.php`** — Added `@deprecated` PHPDoc to
`CHECK_MODE_NONE`; constant is retained to avoid a breaking change and
will be removed in the next major release (7.0.0)

```php
/** @deprecated CHECK_MODE_NONE is unused and will be removed in the next major release (7.0.0) */
public const CHECK_MODE_NONE = 0x00000000;
```

IDEs and static analysis tools (PHPStan, Psalm) will surface the
deprecation warning at call sites.

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Constraint::CHECK_MODE_NONE is defined but never
used</issue_title>
> <issue_description>As it says on the tin: "Constraint::CHECK_MODE_NONE
is defined but never used". Also what would a validator do in a check
mode none?
> 
> This might require some checking and should only be removed in the
next mayor release. However it could be marked as
deprecated</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes #839

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: DannyvdSluijs <618940+DannyvdSluijs@users.noreply.github.com>

Author: Copilot

src/JsonSchema/Constraints/Constraint.php

@@ -11,6 +11,7 @@ abstract class Constraint extends BaseConstraint implements ConstraintInterface
     /** @var string */
     protected $inlineSchemaProperty = '$schema';
 
+    /** @deprecated CHECK_MODE_NONE is unused and will be removed in the next major release (7.0.0) */
     public const CHECK_MODE_NONE =             0x00000000;
     public const CHECK_MODE_NORMAL =           0x00000001;
     public const CHECK_MODE_TYPE_CAST =        0x00000002;