Fix agent enrollment token validation

## Description
Agent enrollment token validation needs improvement for better security and error handling.

## Current Issues
- [ ] Token expiration not properly handled
- [ ] Missing validation for token format
- [ ] No rate limiting on enrollment attempts
- [ ] Insufficient error messages
- [ ] Token reuse prevention not implemented

## Required Fixes
- [ ] Add proper token format validation
- [ ] Implement token expiration checking
- [ ] Add rate limiting for enrollment endpoints
- [ ] Improve error messages and logging
- [ ] Add token usage tracking
- [ ] Implement token revocation

## Steps to Reproduce
1. Try to enroll with expired token
2. Attempt enrollment with invalid token format
3. Make multiple rapid enrollment attempts

## Expected Behavior
- Clear error messages for invalid tokens
- Proper handling of expired tokens
- Rate limiting to prevent abuse
- Secure token validation

## Priority: High
**Type**: Bug
**Component**: Agent, API
**Severity**: Medium