Fix the missing extension fixture for CLI 0.2.3

[sanka-change-bot]

Summary

The real CLI acceptance test treated sanka/drf-to-fastapi as unavailable. The CLI automatically configures the official marketplace, where this extension is published, so that assertion attempts a wheel download instead of receiving SANKA_EXTENSION_NOT_FOUND. Use a deliberately nonexistent extension ID to exercise the intended failure envelope without installing an extension.

Audit

• Requester: @haegwan
• Authoring identity: Sanka Change Bot
• Head SHA: c56a26d2ecec355b80c94cc77b195485bb65792d
• Risk: low
• Deploy mode: none
• Production surface: SDK tests only

Validation

• python3 "$SANKA_CLI_VALIDATION_ROOT/run_worktree_check.py" python -> passed
• Official PyPI sanka-cli[mcp]==0.2.3, wheel SHA256 d7565a075d1865d952e73b73780b7dfeb2e5513bc2e2d58f0ff6e3436b1c8411. All 28 sync, async, protocol, and real subprocess tests pass with warnings treated as errors. The final acceptance run used isolated configuration, the null keyring, blocked Python internet sockets, and Git restricted to file transport with the official marketplace URL rewritten to a local snapshot at extensions commit a44ab1794924d45e8eeaeec456ce5c28c1c17d56. No application API calls or customer migrations ran. -> reported passed
• No-local-check reason: N/A
• Required GitHub checks must complete before landing.
• Valid review findings must be addressed before landing.

Change Safety

• Data or migration impact: No production data or generated SDK changes; one test fixture string only.
• Rollback: Revert this test fixture change.
• Monitoring: Repository deployment helper smoke checks.

Approval And Deployment Authorization

• The PR is ready for final human review; no intended code changes remain.
• The listed head SHA is the exact revision being presented for approval.

By approving this PR, the reviewer authorizes merge of the exact approved revision. This repository has no production deployment in this flow. A later push requires a new approval.