Restore the outer backend after nested runs

[OskarEichler]

Summary

Save and restore the previous thread-local backend around run, including exceptions.

Reproduction and verification

Run a Local backend inside another Local backend and inspect Backend.current after the inner call. Baseline returns nil instead of the still-running outer backend; the candidate restores the outer instance on both success and error.

• Focused branch against master 28e04dc73de911374a23c4fc3fd5ef34012dcd63; installed release is 1.25.1 (e35241b7a3acdf3df4d70442b4a2e69bbcfff155).
• Relevant existing tests pass on this isolated branch. Combined release candidate passes 218 unit tests / 3,405 assertions and nine local functional tests / 13 assertions, plus 12 focused checks, 8,507 model assertions and 22 native local path checks. Combined totals cover this and the other focused changes, not this PR alone.
• No test/spec files added or changed; focused checks are external scratch fixtures. Docker/SSH functional tests were not run. A wrapper disables Docker startup and after-run teardown during the unit/local suites.
• Original RuboCop 0.52.1 fails with Ruby 4/Psych argument compatibility. With an external compatibility shim, changed files pass lint; full lint retains an unrelated existing Lint/BlockAlignment cop error in nil_cache.rb. No full green-lint claim.
• Package build preserves 1.25.1, Ruby >=2.5, dependency metadata and all 96 package paths. Ruby 2.5 syntax parsed by upstream lint tooling; no older-runtime or remote-platform execution claim.

Breaking changes / compatibility / limitations

No intended breaking changes. Outside the outermost run, Backend.current is still nil; the current inner backend remains visible while its block runs. No new per-thread configuration API.

No production access, real SSH sessions, deployment or concurrency increase.