Document VS Code extension and expand test coverage

[clavery]

Summary

• New `/vscode-extension/` docs section (overview with Developer Preview banner, installation with dynamic VSIX download link, configuration reference, feature tour) parallel to the MCP Server section. Sidebar/nav wired up; `ide-integration.md` cross-links so it stays focused on Prophet/IntelliJ.
• Replace the placeholder activation test with a real Extension Host integration suite (`src/test/integration/activation.test.ts`) that asserts activation, command-registration parity with `package.json`, declared debug type, and view focus-command registration.
• Fix the long-standing `tsconfig.test.json` `rootDir: src/test` trap. Tests now import production modules directly (deleted the `sandbox-clone-helpers.ts` mirror).
• New unit tests for `WebDavMappingsProvider` and `SandboxConfigProvider` (no `vscode` import where avoidable). 22 → 66 passing.
• Extend `ci-vs-extension.yml` path filter to also fire on SDK changes (extension bundles the SDK via esbuild, so SDK regressions can break activation) and lockfile/workspace catalog changes; cache `.vscode-test` binary.
• Bumped `@vscode/vsce` floor to `^3.9.1`; added `c8` for coverage (no thresholds yet — Extension Host runs out-of-process so reported coverage is a low-bound).
• Doc-only changeset (`@salesforce/b2c-dx-docs: patch`).

Notes for reviewers

• Screenshots are placeholders. Each `.png` under `docs/vscode-extension/images/` is a 1×1 PNG so the build resolves; every image site has a `` HTML comment immediately above it. Grep for that marker to find every slot awaiting a real image.
• VSIX download link is dynamic. `docs/vscode-extension/release.data.ts` is a VitePress build-time data loader that lists GitHub releases, picks the newest `b2c-vs-extension@X.Y.Z`, and exports `{ version, vsixDownloadUrl, ... }`. On any failure (rate-limit, no releases yet, parse error) it returns `unavailable: true` and the page falls back to a "Browse releases" link. This means the docs build never breaks because of a missing release.
• Release/docs cadence: the floating `b2c-vs-extension@latest` git tag is moved on every VSCE publish but the docs workflow won't auto-rebuild on that — so a freshly published VSCE may briefly trail by one version until the next docs build. Adding a docs-rebuild trigger on `b2c-vs-extension@*` tag pushes is a reasonable separate follow-up.
• CI coverage: `ci-vs-extension.yml` now also fires on SDK changes and caches the VS Code binary (~150 MB). Marginal cost on SDK PRs is ~90s once the cache warms.

Test plan

• CI: `ci-vs-extension.yml` runs (66 tests pass under `xvfb-run` on Linux); `ci.yml` runs typecheck + lint for the extension; main CI green for SDK and other packages.
• Local: `pnpm --filter b2c-vs-extension run test` → 66 passing.
• Local: `pnpm run docs:build` → clean. Open `docs/.vitepress/dist/vscode-extension/installation.html` and confirm the dynamic VSIX download link renders (or fallback if API was unavailable).
• Local: `pnpm run lint:agent && pnpm run typecheck:agent` → clean.
• Visual review of the four new doc pages — install/configuration/features structure, sidebar wiring, top-nav "VS Code" entry.
• Future: replace placeholder PNGs with real screenshots.