When trying to connect to
http://example.com, an HTTP connection is established, not HTTPS and the redirect is unsuccessful.
One issue that you seem to have is that there are no entries for http://example.com in your Apache virtual host configuration. The www prefix is technically a subdomain of example.com and is therefore considered a separate site for resolution. Excluding any issues with Squid, a simple fix would be to modify your virtual hosts to account for the different variations of example.com with the ServerAlias directive:
# Redirect http://example.com and http://www.example.com
# to https://www.example.com
<VirtualHost *:80>
ServerName example.com
ServerAlias www.example.com
Redirect permanent / https://www.example.com
</VirtualHost>
# Serve https://example.com and https://www.example.com
# from the same DocumentRoot
<VirtualHost *:443>
ServerName example.com
ServerAlias www.example.com
DocumentRoot /usr/www/htdocs
SSLEngine On #etc...
</VirtualHost>
Note that you could also redirect https://example.com to https://www.example.com:
# Redirect http://example.com and http://www.example.com
# to https://www.example.com
<VirtualHost *:80>
ServerName example.com
ServerAlias www.example.com
Redirect permanent / https://www.example.com
</VirtualHost>
# Redirect https://example.com to https://www.example.com
<VirtualHost *:443>
ServerName example.com
Redirect permanent / https://www.example.com
SSLEngine On #etc...
</VirtualHost>
# Serve https://www.example.com
<VirtualHost *:443>
ServerName www.example.com
DocumentRoot /usr/www/htdocs
SSLEngine On #etc...
</VirtualHost>
You can read more about name-based virtual hosts here.