fix(session): include available sessions in --resume error for invalid identifier

[Zahed-Riyaz]

Summary

When --resume is given an identifier that doesn't match any session (e.g. --resume 99 when only 3 sessions exist, or a typo'd UUID), the CLI now prints a compact list of available sessions directly in the error output before exiting. Previously the user had to run a separate --list-sessions command to find valid identifiers.

Details

Before:

Error resuming session: Invalid session identifier "99".
  Use --list-sessions to see available sessions, then use --resume {number}, --resume {uuid}, or --resume latest.

After:

Error resuming session: Invalid session identifier "99".

Available sessions for this project:
  1. Fix bug in auth (2 days ago)
  2. Refactor database schema (5 hours ago)
  3. Update documentation (Just now)

Use --resume 1, --resume 2, --resume 3, or --resume latest.

Implementation:

• Extended SessionError.invalidSessionIdentifier(identifier, sessions?) to accept an optional SessionInfo[]. When provided, it formats a compact inline summary (index, display name truncated to 60 chars, relative time) and emits valid --resume N hints.
• SessionSelector.findSession() already has sortedSessions in scope when it throws — the only change needed there is passing it to the error factory.
• Output is capped at 10 entries; if more exist, a Run --list-sessions for the full list. note is appended.
• Callers that don't pass sessions (e.g. generic error paths) continue to receive the original fallback message — no breaking change.

Related Issues

Related to google-gemini#21429 (follow-up: this PR addresses the UX gap left open after that fix).

How to Validate

1. Start the CLI in a project directory that has at least one session (gemini then exit).
2. Run gemini --resume 99 (or any out-of-range number). Verify the error output lists available sessions with indices and relative times, and ends with valid --resume N hints — without needing a separate --list-sessions invocation.
3. Run gemini --resume not-a-real-uuid. Confirm the same inline list appears.
4. Run gemini --resume (no value) in a directory with sessions — should still resolve to latest as before (no change to that path).
5. Run gemini --resume latest in an empty project — should still start a fresh session with a warning (no change to that path).
6. Run gemini --list-sessions — output and behavior unchanged.

Pre-Merge Checklist

• Updated relevant documentation and README (if needed) — no doc changes required; behavior is self-describing
• Added/updated tests (if needed) — added 8 new tests in sessionUtils.test.ts covering unit-level formatting and integration via SessionSelector.findSession
• Noted breaking changes (if any) — none; sessions parameter is optional, all existing call sites unaffected
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker