fix: support Facebook SDK 18 and fix $FacebookSDKVersion override (#57)

* fix: support Facebook SDK 18 and fix $FacebookSDKVersion override

- widen default FBSDKCoreKit constraint from `~> 17.0.2` to `>= 17.0.2, < 19.0` to allow Facebook SDK 18.x
- fix `$FacebookSDKVersion` override crashing pod install (missing `$` referenced undefined constant Pod::FacebookSDKVersion)
- splat the version via `*Array(...)` so both string overrides and the default range are passed to s.dependency

Scanned-by: gitleaks 8.29.0

* chore: format default Facebook SDK version log for array constraint

Join the version requirement array so the install log prints
`>= 17.0.2, < 19.0` instead of the raw Ruby array literal.

Scanned-by: gitleaks 8.29.0

* fix: ignore nil or empty $FacebookSDKVersion override

Guard the override on a non-empty value so a nil/blank
$FacebookSDKVersion (e.g. from a missing env var) falls back to the
default `>= 17.0.2, < 19.0` range instead of dropping the version cap.

Scanned-by: gitleaks 8.29.0

Author: vgupta98

Rudder-Facebook.podspec

@@ -2,7 +2,7 @@ require 'json'
 
 package = JSON.parse(File.read(File.join(__dir__, 'package.json')))
 
-facebook_sdk_version = '~> 17.0.2'
+facebook_sdk_version = ['>= 17.0.2', '< 19.0']
 rudder_sdk_version = '~> 1.12'
 deployment_target = '12.0'
 facebook_app_events = 'FBSDKCoreKit'
@@ -25,11 +25,11 @@ Pod::Spec.new do |s|
     s.source_files = 'Rudder-Facebook/Classes/**/*'
     s.ios.deployment_target = deployment_target
 
-    if defined?($FacebookSDKVersion)
+    if defined?($FacebookSDKVersion) && !$FacebookSDKVersion.to_s.empty?
       facebook_sdk_version = $FacebookSDKVersion
-      Pod::UI.puts "#{s.name}: Using user specified Facebook SDK version '#{FacebookSDKVersion}'"
+      Pod::UI.puts "#{s.name}: Using user specified Facebook SDK version '#{$FacebookSDKVersion}'"
     else
-      Pod::UI.puts "#{s.name}: Using default facebook SDK version '#{facebook_sdk_version}'"
+      Pod::UI.puts "#{s.name}: Using default facebook SDK version '#{Array(facebook_sdk_version).join(', ')}'"
     end
 
     if defined?($RudderSDKVersion)
@@ -40,5 +40,5 @@ Pod::Spec.new do |s|
     end
 
     s.dependency 'Rudder', rudder_sdk_version
-    s.dependency facebook_app_events, facebook_sdk_version
+    s.dependency facebook_app_events, *Array(facebook_sdk_version)
 end