Add smoke test for shell completion script behavior

[Vaishnav88sk]

Problem

Changes to the completion script may break tab completion behavior without being detected in CI.

Proposal

Add a minimal smoke test that validates:

• Script can be sourced/loaded successfully
• Basic completion output contract works as expected
• Representative commands return expected completion candidates

Acceptance criteria

• Test runs in CI (Linux shell environment)
• Covers at least one happy-path completion scenario
• Fails with actionable output if the completion script breaks

Contribution

I’d be happy to work on this issue and contribute a PR if this approach sounds good.