#2263 test(bot): add spec coverage for /session

Summary

packages/bot/src/functions/music/commands/session.ts (258 LOC) had no spec while every sibling music command does. /album shipped 100% broken for every text query and stayed that way until #2053 precisely because it had no spec either (#2050) - same risk class here.

Coverage added

• save: no-active-queue warning, name lowercasing, the service's duplicate/invalid/limit refusal, and success
• list: empty state and the formatted per-session line
• delete: found/not-found, and that it skips the voice-channel guard restore needs
• restore: voice-channel guard, reusing vs creating a queue, the queueConnect failure path, the zero-restored-tracks not-found case, and success

Test plan

• npx jest src/functions/music/commands/session.spec.ts - 16/16 pass
• Verified each assertion actually fails against a mutated implementation (forced branches to always/never take the no-queue and delete-found paths, forced restore to always create a fresh queue) before confirming it passes against the real code - the same check that would have caught #2050
• npx tsc -p packages/bot --noEmit clean
• No production code changed - test-only PR

Closes #2057

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Summary by cubic

Adds the missing spec for the /session music command so it has the same regression protection as sibling commands, closing #2057. Test-only PR with no production code changes.

Coverage

• save: warns without an active queue, lowercases the session name, handles the service's duplicate/invalid/limit refusal, and reports success.
• list: covers the empty state and the formatted per-session line.
• delete: covers found, not-found, and that it skips the voice-channel guard restore needs.
• restore: covers the voice-channel guard, queue creation vs reuse, the queueConnect failure path, the zero-restored-tracks not-found case, and success.
• Every assertion was verified to fail against a mutated implementation before passing against the real code.