fix: support Facebook SDK 18 and fix $FacebookSDKVersion override

[vgupta98]

Description

Updates the FBSDKCoreKit dependency handling in Rudder-Facebook.podspec to support Facebook SDK 18.x and to fix a crash in the $FacebookSDKVersion override path.

Previously the pod pinned the Facebook SDK to ~> 17.0.2, which only allows 17.0.x. Apps that use Facebook SDK 18.x could not install the pod because of a version conflict. Additionally, setting the $FacebookSDKVersion global in a Podfile crashed pod install due to a missing $ in a log statement.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactor/optimization

Implementation Details

• Widened the default Facebook SDK version constraint from ~> 17.0.2 to ['>= 17.0.2', '< 19.0'], allowing all 17.x and 18.x releases.
• Fixed the $FacebookSDKVersion override log line: it referenced #{FacebookSDKVersion} (missing the $), which Ruby resolved as the undefined constant Pod::FacebookSDKVersion and crashed pod install with uninitialized constant Pod::FacebookSDKVersion. Now uses #{$FacebookSDKVersion}.
• Passed the version requirement to s.dependency via *Array(facebook_sdk_version), so both a user-supplied string override and the default multi-requirement array are forwarded correctly.

Checklist

• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation (if appropriate).
• I have ensured that my code follows the project's code style.
• I have checked for potential performance impacts and optimized if necessary.
• I have checked the code for security issues.
• I have updated the changelog (if required).

How to test?

Verified locally against the Example app:

1. Facebook SDK 18 resolves — pod update FBSDKCoreKit installs FBSDKCoreKit 18.1.0 with the new range (previously blocked at 17.0.x).
2. Override no longer crashes — set $FacebookSDKVersion = '~> 17.0' in the Podfile and run pod install; it prints Using user specified Facebook SDK version and installs successfully. On the previous podspec the same step crashed with uninitialized constant Pod::FacebookSDKVersion.
3. Builds against FBSDK 18 — xcodebuild of the Rudder-Facebook-Example scheme against FBSDKCoreKit 18.1.0 completes with BUILD SUCCEEDED; the integration only uses stable App Events APIs unchanged in 18.

Breaking Changes

None. The change only widens the allowed dependency range and fixes the override path; existing installs on 17.x are unaffected.

Maintainers Checklist

• The code has been reviewed.
• CI tests have passed.
• All necessary documentation has been updated.

Screenshots (if applicable)

N/A

Additional Context

Reference implementation: original external contribution in #56. Tracked under Linear SDK-5068.

[Copilot]

Pull request overview

This PR updates the CocoaPods spec for the Rudder Facebook integration to allow installation alongside newer Facebook SDK releases (18.x) and fixes a pod install crash when users override the Facebook SDK version via $FacebookSDKVersion in their Podfile.

Changes:

• Widened the default FBSDKCoreKit version constraints to allow Facebook SDK 17.x and 18.x (>= 17.0.2, < 19.0).
• Fixed the $FacebookSDKVersion override log interpolation so it doesn’t reference an undefined constant.
• Updated dependency declaration to forward either a string override or an array of requirements correctly (*Array(facebook_sdk_version)).

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[dciccale]

Re-reviewed the latest changes. The nil/empty $FacebookSDKVersion fallback is addressed, and validation is green.