fix: don't prompt when all workspace folders share one Bazel workspace

[omri-steinmetz]

Problem

In a multi-root VS Code workspace, BazelWorkspaceInfo.fromWorkspaceFolders() unconditionally calls vscode.window.showWorkspaceFolderPick(). When it runs without a user gesture — e.g. the Bazel target completion-provider refresh right after activation — the pick resolves to undefined and the extension logs:

[error] Failed to find a Bazel workspace

This happens even when every workspace folder belongs to the same Bazel workspace (a common monorepo layout: a root folder plus per-package/per-service subfolders in a .code-workspace). There is nothing for the user to disambiguate, and no setting to suppress the error.

Related: #446 covers the non-Bazel-repo case; this covers the multi-root single-Bazel-workspace case.

Fix

Resolve each workspace folder to its Bazel workspace root first (via the existing fromWorkspaceFolder), then:

• 0 distinct Bazel workspace roots → return undefined silently (no folder is a Bazel workspace; also reduces noise for the "Failed to find a Bazel Workspace file" on non bazel repos that include BUILD files #446 scenario);
• 1 distinct root → use it without prompting — this fixes the spurious startup error;
• multiple distinct roots → show the folder picker, exactly as before (genuinely ambiguous).

Testing

• npm run typecheck, eslint, prettier --check, and npm run build pass.
• xvfb-run -a npm run test:unit: 123 passing, 1 failing — the same pre-existing lcov parser / function coverage details failure occurs on unpatched master in the same environment (Rust demangling difference), unrelated to this change.
• Manually verified against a 20-folder multi-root workspace whose folders all resolve to one Bazel workspace root: the activation-time error no longer fires, and target queries use the shared root.

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