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/constants.go

@@ -10,6 +10,7 @@ var (
 
 	// Package validation errors
 	ErrPackageNameHasSpaces = errors.New("package name cannot contain spaces")
+	ErrReservedVersionString = errors.New("version string 'latest' is reserved and cannot be used")
 
 	// Remote validation errors
 	ErrInvalidRemoteURL = errors.New("invalid remote URL")