fix: use task workspace for codebase search

[WebMad]

Summary

• Pass the already resolved task workspace to the code index manager registry instead of allowing active-editor workspace selection.
• Keep the existing fallback for empty/whitespace task working directories.
• Add regression coverage using the real registry and mocked managers, verifying root B is searched while the active editor belongs to root A, query/path forwarding, result output, fallback selection, and missing workspace handling.

Stacked PR

Depends on Zoo-Code-Org#1622.
Branched from its exact head: 8637e48.
This PR targets the parent branch in WebMad/Zoo-Code so the diff contains only this bugfix and its tests, not the parent refactor. After the parent merges, this change needs an upstream-targeted PR (retarget/recreate as appropriate; the current fork PR cannot directly change its base repository). No changes to PR Zoo-Code-Org#1622 or Zoo-Code-Org#1628 and no workspace-scopes refactoring.

Verification

• Observed the regression test fail before the production change: the task-root manager received zero search calls.
• After the one-line fix: focused regression and registry suites passed (15 tests).
• Relevant code-index suite plus regression tests: 27 files, 619 tests passed.
• Changed-file ESLint with prune-suppressions and max-warnings=0 passed; suppression data unchanged.
• Prettier completed and git diff whitespace check passed.
• Extension TypeScript check passed.
• Normal commit/push hooks passed repository lint and type-check tasks (11 tasks each, 10 cached).

Non-blocking environment warnings: Node v24.7.0 differs from required 22.23.1; Prettier reports an existing unknown ignore option. Full repository tests and extension-host E2E were not run.