test: add package installation integration tests #1044

Parent Issue

Part of #1039

Problem

Package manager tests in `chroot-package-managers.test.ts` only query registries (`pip index versions`, `npm view`, `cargo search`, `gem search`) but never install packages (`pip install`, `npm install`, `cargo add`). This is the most common real-world operation.

Acceptance Criteria

• Add integration tests to `tests/integration/chroot-package-managers.test.ts` that verify actual package installation:
  • `pip install requests` (or similar small package) through the proxy with `pypi.org` + `files.pythonhosted.org` allowed
  • `npm install chalk` (or similar) through the proxy with `registry.npmjs.org` allowed
  • `cargo add serde` or `cargo build` with dependencies through the proxy with `crates.io` + `static.crates.io` + `index.crates.io` allowed
• Each test should verify the package was actually installed (e.g., `pip show requests`, `node -e "require('chalk')"`, `cargo build` succeeds)
• Tests should be in a separate describe block (e.g., "Package Installation") to distinguish from existing query tests
• Use the batch runner pattern where possible for efficiency

References

• `tests/integration/chroot-package-managers.test.ts`: Existing tests to extend
• `tests/fixtures/batch-runner.ts`: Batch runner for grouping commands

Closes #1039 (when combined with other sub-issues)