[codex] Fix manual iOS build workflow

[MichaelSpece]

Summary

Fix the manual Builds workflow so flutter build ios --no-codesign passes on GitHub-hosted macOS runners again.

Motivation

A manual baseline run against clean upstream main reproduced the current iOS build failure before this change:

• Baseline run: https://github.com/MichaelSpece/mobile/actions/runs/27955673530
• Failure: connectivity_plus-7.1.1 references NWPath.isUltraConstrained, which is unavailable on the macos-latest runner image used by the workflow.

This is separate from #3316 and from the requested installable Android artifact work in #3337. It makes the existing manual build workflow healthy first, so #3337 can later add APK artifact upload without being mixed with an iOS runner repair.

Changes

• Run the manual Builds workflow on macos-26.
• Guard the Crashlytics upload-symbols phase so no-codesign/manual CI builds skip symbol upload when the helper script is absent.

Validation

Validation run on this branch: https://github.com/MichaelSpece/mobile/actions/runs/27959590265

• Build ios --no-codesign: passed
• Build appbundle --debug: passed