fix: find re2 headers

[ncooke3]

Address re2 header related nightlies #15607 (comment)

Gemini Explanation:

1. CocoaPods Cleanup: When CocoaPods installs a pod, it cleans the source directory by removing any files that are not explicitly declared in the podspec (via source_files, resources, preserve_paths, etc.).
2. Missing Headers: Before your change, the re2 headers located in Firestore/third_party/re2 were not listed in source_files (since they are excluded or not matched) and were missing from preserve_paths. As a result, CocoaPods deleted them.
3. Broken Search Path: Although HEADER_SEARCH_PATHS correctly pointed to /Firestore/third_party/re2, the compilation failed because the files had been removed from the disk.
4. The Fix: Adding 'Firestore/third_party/re2/**/*.h' to preserve_paths explicitly tells CocoaPods to keep these files. This ensures they exist on disk during the build, allowing the compiler to resolve #include re2/re2.h.

Testing

• Successfully staged to SpecStaging (firebase/SpecsStaging@d4a9559)
• manual prerelease: https://github.com/firebase/firebase-ios-sdk/actions/runs/20663912566
• manual release: https://github.com/firebase/firebase-ios-sdk/actions/runs/20663904925
• symbol collision: Merge main then re-trigger

Fixes #15607

#no-changelog

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[paulb777]

LGTM

Please test locally or trigger CI for one of the failing nightlies to confirm.

[ncooke3]

I'm going to merge this and see how nightlies respond. I waited for manual (pre)release_cocoapods workflows but realized that I shouldn't have since the tags they were testing do not include this fix. I was able to successfully stage the fixed spec in this PR to SpecsStaging.

Going to merge and move tags.