fix: preserve completion cursor boundaries

[codeforester]

Summary

• Make the generated completion adapter's cursor boundary invariant explicit: an end cursor gets an empty current word, while an in-range cursor preserves the existing word slice.
• Add coverage for exact nested commands at the end cursor, out-of-range cursors, and unset completion variables under set -u.

Issue

Fixes #527

Validation

• bats lib/bash/cli/tests/lib_cli.bats (40 tests passed).
• ./tests/validate.sh (682 tests passed; artifact, release, concurrency, and quality contracts passed).
• shellcheck --shell=bash --severity=warning lib/bash/cli/lib_cli.sh lib/bash/cli/tests/lib_cli.bats
• bash -n lib/bash/cli/lib_cli.sh
• git diff --check

API Impact

No command declaration, parser, handler, or option-resolution semantics changed. This preserves the documented nested-command completion behavior at the cursor boundary and makes the invariant regression-tested.