fix: make SSH wrapper compatible with remote fish shells

[oz-for-oss]

Closes #6548

Summary

• Route the SSH wrapper's remote POSIX bootstrap payload through sh instead of letting the remote login shell parse it directly.
• Apply the same wrapper path from local Bash, Zsh, and Fish bootstrap helpers.
• Preserve the existing SSH hook payload, Bash/Zsh bootstrap paths, and fallback for other remote shells.

Validation

• bash -n app/assets/bundled/bootstrap/bash_body.sh
• zsh -n app/assets/bundled/bootstrap/zsh_body.sh
• fish -n app/assets/bundled/bootstrap/fish.sh
• Fish parsing smoke test for the encoded sh wrapper command
• cargo fmt --check
• CARGO_BUILD_JOBS=1 cargo clippy --workspace --all-targets --all-features --tests -- -D warnings

Notes

• Focused Rust bootstrap tests were attempted, but the sandbox killed the test build with SIGKILL before execution. The clippy command completed successfully with a single build job.