fix: Xcode/OS matrices in reusable SPM and CP workflows

[ncooke3]

Both reusable workflows will test against:

1. Xcode 16.2 on macOS 14 for iOS only.
2. Xcode 16.2 on macOS 15 for iOS only.
3. Xcode 16.4 on macOS 15 for all platforms.
4. Xcode 26.1 on macOS 26 for all platforms.

Contexts:

• https://docs.github.com/api/article/body?pathname=/en/actions/reference/workflows-and-actions/workflow-syntax.md
• https://docs.github.com/api/article/body?pathname=/en/actions/how-tos/reuse-automations/reuse-workflows.md

#no-changelog

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[ncooke3]

Fixes a bug that reappeared in this PR where the quickstart script that updates the SPM dependency to point to this PR fails because Github creates a "ghost" commit that does not exist outside of the PR's CI context. This is a problem since the quickstart needs to point to a valid commit on the remote of the Firebase repo. This solution seems to work so far.

[ncooke3]

/gemini review

[gemini-code-assist]

Code Review

This pull request updates the Xcode and OS matrices for reusable CI workflows. The changes involve updating simulator versions and build flags in build.sh to support newer Xcode versions, and improving the logic in scripts/setup_quickstart.sh to correctly obtain the commit SHA for pull requests. A debug log was also removed. The changes are sound and improve the CI setup. I've added one suggestion to make the script that retrieves the commit SHA more robust.

[andrewheard]

😆 I'm not sure we want to clean this up. I like it.

[ncooke3]

I removed the macos-15 x Xcode 16.2 testing since it seems to be flaky (I think due to iOS 18.3 simulator networking issues).

I will let the current CI finish and then try moving to Xcode 26.2 to smoke test that version.

[ncooke3]

Going to do a follow-up with Xcode 26.2.