Integration tests against IRIS containers for release-preparation PRs

[isc-klu]

Reading order

1. test-fixtures/README.md — the design: cases, checks, containers, how to run locally.
2. src/test/cases.ts — that matrix as data; src/test/suite/extension.test.ts — the checks.
3. src/test/runTest.ts and .github/workflows/prepare-release.yml — how they get run.

Companion of intersystems-community/vscode-objectscript#1870; both repos run the same matrix, each installing the other extension's Marketplace release.

What changes

• New prepare-release.yml workflow runs the suite against IRIS containers on prepare-* PRs and manual dispatch.
• The placeholder npm test (a single assert.ok("All good"), run on every push) is replaced, and the step leaves main.yml and prerelease.yml.
• @vscode/test-electron 2.5 → 3.1 (2.x looks for a binary current VS Code no longer ships on macOS), plus skipLibCheck for its typings.

Decisions worth knowing

• Podman only. The workflow removes the runner's Docker Compose (the ObjectScript extension tries it first), and runTest.ts pins PODMAN_COMPOSE_PROVIDER=podman-compose (Docker Desktop's plugins hijack podman compose).
• Isolation. Fresh --user-data-dir per case; the suite asserts it is running the build under test, not the Marketplace copy the ObjectScript extension depends on.
• Lapsed sessions. 10 s /api/atelier timeout; every check reruns as the first request after it lapses.

Verified by mutation

Reverting #347 fails the check that a password stored in settings reaches API consumers; reverting #352 fails the -anonymous namespace listings.

Known failing on CI

All against the Marketplace ObjectScript 3.8.5; they clear once 3.8.6 ships (runTest.ts installs it with --pre-release).

• Always: the two clientSide-os-docker cases (need its Podman support) and the four -inactive cases' flip test (Fix deletes not syncing after activating the connection without reloading the window vscode-objectscript#1872).
• Intermittently, on the first request after an authenticated session lapses (Fix connection check prompting/disabling on a merely-stale session vscode-objectscript#1864): serverSide-sm-named › "OS lists the folder after the session times out" throws Cannot read properties of undefined (reading 'includes'); active -named cases › "round-trips after the session times out" don't propagate the delete.

Not covered

• The authentication provider's prompt and secret storage: they need typed input or seeded secrets, and vscode.authentication.getSession from a test hits VS Code's consent dialog.
• Incorrect account detected #353 (fix split across both extensions, no self-contained repro) and OAuth2 OAuth2 VSCode #335 (needs an identity provider).