Ban 'latest' as a reserved version string (#413)

## Summary
- Adds validation to prevent servers from being published with version
string "latest"
- "latest" is now a reserved keyword that cannot be used as a version
string

## Implementation
- Added `ErrReservedVersionString` error constant
- Created `validateVersion` function to check for reserved version
strings
- Integrated validation into the package field validation flow
- Added comprehensive test coverage

---

This is so we can potentially support things like
https://github.com/modelcontextprotocol/registry/discussions/333 in
future more easily, as I imagine we might want to.

internal/validators/validators_test.go

@@ -193,6 +193,30 @@ func TestValidate(t *testing.T) {
 			},
 			expectedError: validators.ErrPackageNameHasSpaces.Error(),
 		},
+		{
+			name: "package with reserved version 'latest'",
+			serverDetail: apiv0.ServerJSON{
+				Name:        "com.example/test-server",
+				Description: "A test server",
+				Repository: model.Repository{
+					URL:    "https://github.com/owner/repo",
+					Source: "github",
+				},
+				Version: "1.0.0",
+				Packages: []model.Package{
+					{
+						Identifier:      "test-package",
+						RegistryType:    "npm",
+						RegistryBaseURL: "https://registry.npmjs.org",
+						Version:         "latest",
+						Transport: model.Transport{
+							Type: "stdio",
+						},
+					},
+				},
+			},
+			expectedError: validators.ErrReservedVersionString.Error(),
+		},
 		{
 			name: "multiple packages with one invalid",
 			serverDetail: apiv0.ServerJSON{