feat: add datamachine_oauth_callback_url filter hook

[chubes4]

Summary

• Adds datamachine_oauth_callback_url filter to BaseAuthProvider::get_callback_url() so extension plugins can customize the OAuth redirect URI per provider
• Applies the same filter to the legacy datamachine_get_oauth_callback_url() helper for consistency
• Adds 3 tests covering filter behavior, slug passthrough, and provider isolation

Context

Closes #1041. Intelligence registers a WordPress.com OAuth provider (client_id 128117, shared with mcp-context-a8c) that expects /oauth/callback as the redirect path, not /datamachine-auth/wpcom/. This filter lets Intelligence hook in cleanly:

add_filter( 'datamachine_oauth_callback_url', function ( $url, $slug ) {
    if ( 'wpcom' === $slug ) {
        return site_url( '/oauth/callback' );
    }
    return $url;
}, 10, 2 );

Default behavior is unchanged — the filter passes through if no one hooks it.

Testing

All 19 BaseAuthProvider tests pass (16 existing + 3 new).