[Bug]: [Presets] dstack preset delete cannot delete interrupted or failed creations

[peterschmidt85]

Steps to reproduce

1. Start a preset creation: dstack apply -f preset.dstack.yml. Preset configurations do not support -d, so it holds the terminal.
2. Stop it before it finishes: Ctrl+C and confirm the prompt, or dstack preset stop <id> -y from a second terminal. A creation that failed on its own works the same way.
3. Confirm the session is listed: dstack preset list -a.
4. Delete it: dstack preset delete <id> -y, or by name if it has one.

Actual behaviour

$ dstack preset list -a
 NAME           ID        BASE             CONSTRAINTS    BENCHMARK  STATUS             SUBMITTED
 smoke-qwen06b  8a2d894e  Qwen/Qwen3-0.6B  io=256/64 c=4             interrupted (0/2)  8 mins ago

$ dstack preset delete 8a2d894e
Preset '8a2d894e' does not exist

$ dstack preset delete smoke-qwen06b
Preset 'smoke-qwen06b' does not exist

Neither the ID nor the name works, and --base and --repo skip these sessions too. The only way to remove one is rm -rf ~/.dstack/presets/<id>, which leaves the /tmp/dpe-* workspace alias symlink behind (workspace.py:145-153, removed only by remove_agent_workspace).

Reproduced on master 0dee238d4 with an interrupted and a failed session.

Expected behaviour

Interrupted and failed creations delete like any other preset:

$ dstack preset delete 8a2d894e
Deleted preset 8a2d894e for Qwen/Qwen3-0.6B

A creation that is still trialing or verifying is refused because it is in use, the way dstack refuses to delete a fleet or volume that is in use, rather than reported as non-existent:

$ dstack preset delete 8a2d894e
Failed to delete preset 8a2d894e. Preset creation is in use. Stop it with `dstack preset stop 8a2d894e`

Cause

_delete resolves its argument with find_by_id_or_name (preset.py:363), which only matches ~/.dstack/presets/<id>/preset.yml (store.py:67-69, store.py:38). That file is written only after a creation is verified (create.py:682), so every creation that never got there is invisible to delete.

Two more places need attention for a complete fix:

• PresetStore.delete refuses a directory without preset.yml (store.py:128-129), so resolving the reference is not enough on its own.
• _delete ignores the return value of store.delete and prints Deleted ... unconditionally (preset.py:386-388), so a partial fix would report success while removing nothing.

For reference, dstack preset get already reads unfinished sessions through _get_unfinished_preset (preset.py:391-406), and stop, logs, and resume resolve them with resolve_session_ref (session.py:491-499).

The on-disk statuses are running, interrupted, success, and failed (models/preset_agent.py:69); trialing and verifying are display labels (output.py:14-19). "In use" therefore has to mean session_process_alive (session.py:359-367), the predicate list already uses, or a stale running left by a killed CLI would stay undeletable, which is this same bug again.

Open questions

• Should delete reconcile first? _delete never calls _reconcile (unlike preset.py:222, 260, 333), so a detached session whose agent already finished would be deleted rather than finalized.
• Should --base and --repo cover unfinished sessions? _list already matches them via _session_matches_model (preset.py:480-492).
• Name precedence between a verified preset and a session's claimed name is unspecified.
• Sessions interrupted without dstack preset stop (killed CLI, reboot) may still have runs on the server; _delete builds no API client today (preset.py:359-360).
• mkdocs/docs/reference/cli/dstack/preset.md:135 and mkdocs/docs/concepts/presets.md:281-288 describe only the verified case and need updating with the fix.
• Compounds with [Bug]: [Presets] Any failure during resume marks the creation permanently failed #4164: a resume-time exception marks a session failed, which is exactly a state this bug makes undeletable.