[sdk] CLI installs duplicate shell completion entries on repeated --install-completion

[Riddhima8]

Environment

• KFP version:
  N/A (CLI usage, not a deployed Kubeflow Pipelines cluster)

• KFP SDK version:
  main branch (local install from https://github.com/kubeflow/pipelines)

• All dependencies version:
  pip list | grep kfp

Steps to reproduce

1. Install shell completion for bash:
   kfp --install-completion bash

2.Run the same command again:
kfp --install-completion bash

3.Open the shell configuration file:
cat ~/.bashrc

Observe that the shell completion initialization line has been appended
multiple times.

*Expected result
The CLI should check whether the shell completion initialization line already
exists in the target shell configuration file before appending it.

Running kfp --install-completion <shell> multiple times should not result in
duplicate entries in the shell configuration file.

*Materials and Reference
Source code:
sdk/python/kfp/cli/cli.py

*Relevant function:
_install_completion(shell: str)

*Current behavior:
The completion statement is appended unconditionally using file append mode,
which leads to duplicate shell completion entries.

[Riddhima8]

Hi! I’d like to work on this issue if it’s available.
Please let me know if I can be assigned to it — happy to submit a PR. Thank you!