feat(validators): Validate that Server versions are not ranges (#435)

<!-- Provide a brief summary of your changes -->

Validates that server versions are not ranges.


**NB:** This also introduces `validateVersion` as a check on the
top-level `ValidateServerJSON`, as previously we were only checking
version ranges of packages via `validatePackageField` (i.e. `latest`
would have still been a valid top level server version), despite #413's
wording suggesting otherwise.

## Motivation and Context
Solves https://github.com/modelcontextprotocol/registry/issues/415

## How Has This Been Tested?

Tests added in `internal/validators/validators_test.go`

`make check` is clean.

## Breaking Changes
As per @domdomegg's comment, we should
> check nobody has done this in the existing registry data (if they
have, we should explore and understand why people have done this / if
it's valid etc.).

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

docs/reference/server-json/server.schema.json

@@ -72,7 +72,7 @@
           "type": "string",
           "maxLength": 255,
           "example": "1.0.2",
-          "description": "Version string for this server. SHOULD follow semantic versioning (e.g., '1.0.2', '2.1.0-alpha'). Equivalent of Implementation.version in MCP specification. Non-semantic versions are allowed but may not sort predictably."
+          "description": "Version string for this server. SHOULD follow semantic versioning (e.g., '1.0.2', '2.1.0-alpha'). Equivalent of Implementation.version in MCP specification. Non-semantic versions are allowed but may not sort predictably. Version ranges are rejected (e.g., '^1.2.3', '~1.2.3', '>=1.2.3', '1.x', '1.*')."
         },
         "website_url": {
           "type": "string",
@@ -105,7 +105,8 @@
           "type": "string",
           "description": "Package version",
           "example": "1.0.2",
-          "minLength": 1
+          "minLength": 1,
+          "remarks": "Must be a specific version. Version ranges are rejected (e.g., '^1.2.3', '~1.2.3', '>=1.2.3', '1.x', '1.*')."
         },
         "file_sha256": {
           "type": "string",