Fix nil crash in delete_unused_runtimes when a required runtime is missing

[afurm]

Summary

This PR fixes a crash in RuntimeHelper#delete_unused_runtimes that could happen when at least one required runtime is still missing.

Problem

delete_unused_runtimes built needed_simctl_runtimes with map.
If a required runtime had no match, simctl_runtime_matching_needed_runtime? returned nil.
Later, the code called needed_simctl_runtime.identifier, which raised:

NoMethodError: undefined method 'identifier' for nil

Changes

• Updated delete_unused_runtimes to use filter_map instead of map, so unmatched (nil) runtimes are excluded safely.
• Added a regression spec for the mixed case:
  • one required runtime matches
  • one required runtime is missing
  • method does not crash
  • unused runtime deletion still proceeds

Validation

• bundle exec rspec
  • Result: 44 examples, 0 failures
• bundle exec rubocop lib/.../runtime_helper.rb spec/.../runtime_helper_spec.rb
  • Result: no offenses

Risk

Low.
The change is small, isolated, and covered by a regression test.

[nekrich]

Nice catch. Thanks for the PR.