fix(nu): avoid multiline keybinding commands

[mirsella]

Nushell can expose the multiline source passed to ExecuteHostCommand, which causes Atuin's generated command to appear in the command line and history.

Move the search logic into _atuin_search and have the keybindings execute a single-line command. The per-shell token still distinguishes generated commands from manually entered ones, and exact matching keeps those commands out of Atuin history.

The newer commandline edit --accept path remains generated at runtime so the integration still parses on older supported Nushell versions.

Fixes #2900.

Tested with:

• cargo fmt --all -- --check
• cargo +nightly test --locked -p atuin shell --quiet
• nu-check on Nushell 0.93.0 and 0.114.1
• isolated PTY tests for edit, accept, and history behavior

[greptile-apps]

Greptile Summary

Moves Nushell search logic into _atuin_search, leaving keybindings with single-line generated commands while preserving version-specific edit behavior and filtering generated commands from history.

Confidence Score: 5/5

The PR appears safe to merge.

The generated Ctrl-R and up-arrow commands match the two history-filter variants, and version-specific Nushell behavior remains selected at runtime.

Important Files Changed

Filename	Overview
crates/atuin/src/shell/atuin.nu	Refactors search execution and exact history filtering; no concrete defect found.
crates/atuin/src/command/client/init/nu.rs	Updates the up-arrow binding to the new boolean command interface consistently.

_{Reviews (1): Last reviewed commit: "fix(nu): avoid multiline keybinding comm..." | Re-trigger Greptile}