Function validateTokenFormat

validateTokenFormat(token: string): boolean
Validate JWT token format and structure without verification

Performs basic structural validation of a JWT token without cryptographic verification. Checks that the token has the correct three-part format (header.payload.signature) and contains required JWT claims in the header and payload sections.

This function is useful for quick format validation before attempting expensive cryptographic verification. It helps filter out obviously malformed tokens and provides early validation in authentication flows.
Parameters
- token: string
  JWT token string to validate
Returns boolean
True if token has valid JWT structure, false otherwise
Example
```
// Validate token format before verification
const token = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...';
const isValidFormat = validateTokenFormat(token);

if (isValidFormat) {
  // Proceed with cryptographic verification
  const decoded = jwt.verify(token, secret);
} else {
  console.log('Invalid token format');
}
```
Example
```
// Examples of validation results
validateTokenFormat('invalid.token');           // false (only 2 parts)
validateTokenFormat('not.a.jwt.token');         // false (4 parts)
validateTokenFormat('valid.jwt.token');         // true (if properly formatted)
```
- Defined in src/shared/utils/auth.ts:479