Keep stable shell paths in detection and recover stale shell preferences

[thiagohflima]

Description

Fixes #15836.

On macOS, selecting a Homebrew-installed shell (e.g. fish) as the default shell for new sessions silently reverted to the system default (zsh) after every brew upgrade of the shell.

Root cause: shell detection canonicalized /opt/homebrew/bin/<shell> to the versioned Cellar path (dunce::canonicalize in resolve_all_executables / load_fallback_shells), and that versioned path was persisted as the NewSessionShell::Executable preference. A formula upgrade removes the old Cellar directory, so the exact-path match against re-detected shells failed and get_user_preferred_shell silently fell back to SystemDefault.

This PR implements the direction suggested in the issue's triage:

• Detection keeps the discovered (symlink-stable) path for storage and launch; the canonical path is used only as the dedupe key in resolve_all_executables and load_fallback_shells. New preferences persist /opt/homebrew/bin/fish, which survives upgrades.
• get_user_preferred_shell recovers preferences that no longer exact-match a detected shell instead of silently resetting: an existing path is matched by canonical target (covers preferences persisted by older builds, i.e. the rollout transition), and a removed path resolves to the detected known shell of the same type, preferring the install closest to the stale path (multi-install machines).
• Session restore (get_from_shell_launch_data) applies the same recovery to stale snapshot paths before falling back to a custom shell, so restored workspaces also survive an upgrade that happened while Warp was closed.
• Deliberate out-of-catalog executables (existing file matching no detected shell) are unchanged: they still launch via the existing fallback paths.

No UI changes.

Linked Issue

Fixes #15836

• The linked issue is labeled ready-to-spec or ready-to-implement. — filed by this PR's author with a full root-cause analysis and repo-validated repro (repro:high, deemed eligible for auto-implementation); per the "PRs opened without a linked issue" guidance, this PR links it for maintainer readiness review.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes). — not applicable; no UI changes.

Testing

Automated (new regression tests in app/src/terminal/available_shells_tests.rs, all hermetic via VirtualFS):

• test_keeps_stable_symlink_path_for_homebrew_shells — detection stores the stable bin/fish symlink, not the Cellar path, in a simulated Homebrew layout.
• test_recovers_executable_preference_via_canonical_alias — a preference persisted as an old Cellar path still matches while the file exists (Warp-update transition).
• test_recovers_stale_executable_preference_by_shell_type — a removed Cellar path recovers to the detected fish.
• test_recovery_prefers_the_closest_install_prefix — multi-install machines pick the install closest to the stale path.
• test_does_not_recover_existing_unmatched_or_non_shell_preference — out-of-catalog executables keep prior behavior.
• test_get_from_shell_launch_data_recovers_stale_snapshot_path — session restore recovers stale snapshot paths; existing unknown paths still restore as custom shells.
• test_dedupe_symlinks_when_discovering_paths — updated to document that dedupe keys are canonical while stored paths remain the discovered ones.

I don't have a Rust toolchain on this machine, so I lean on CI for ./script/presubmit (fmt/clippy/nextest); I verified the change by close reading and self-review only. Happy to push follow-ups if CI flags nits.

• I have manually tested my changes locally with ./script/run — unable to build locally (no toolchain); the change alters shell resolution logic only and is covered by the tests above, but I understand if a manual ./script/run pass is needed and can arrange one on request.

Screenshots / Videos

Not applicable — no user-visible UI change.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-BUG-FIX: Fixed the default shell preference silently resetting to the system default shell after a Homebrew upgrade of the shell.

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@thiagohflima

Every PR must be linked to a same-repo issue before Oz can review it.

This PR is linked to #15836, but no linked issue is marked ready-to-implement yet. Only repository maintainers apply that label, so please wait for a maintainer to mark the issue. Once it is marked, push a new commit or comment /warp-agent-review to re-trigger review.

See the contribution guidelines for the full readiness model.

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[warp-for-oss]

@thiagohflima

Every PR must be linked to a same-repo issue before Oz can review it.

This PR is linked to #15836, but no linked issue is marked ready-to-implement yet. Only repository maintainers apply that label, so please wait for a maintainer to mark the issue. Once it is marked, push a new commit or comment /warp-agent-review to re-trigger review.

See the contribution guidelines for the full readiness model.

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[thiagohflima]

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[thiagohflima]

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