chore: Add btp cli shell completion for bash, pwsh and zsh

[paulchen5]

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After making any changes to a cask, existing or new, verify:

• The submission is for a stable version or documented exception.
• brew audit --cask --online <cask> is error-free.
• brew style --fix <cask> reports no offenses.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
• Checked the cask was not already refused (add your cask's name to the end of the search field).
• brew audit --cask --new <cask> worked successfully.
• HOMEBREW_NO_INSTALL_FROM_API=1 brew install --cask <cask> worked successfully.
• brew uninstall --cask <cask> worked successfully.

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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes, including zap stanza paths.

Explain the brew style output and let it fix it as brew style requires a brew tap to work.
Verification: CI didn't work previously; now it works; I am not familiar with ruby style code

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[paulchen5]

When running btp enable autocomplete <shell-name> something like this should be added to your shells rc file:

### Begin SAP BTP command line interface autocomplete
eval "$(btp --autocomplete=init:zsh)"
### End SAP BTP command line interface autocomplete

[paulchen5]

Is there a way to check that it works locally?

[paulchen5]

Well I think I was mixing up casks here with formulas. Is there a reason why the cli tool is being counted as a cask?

[bevanjkay]

Suggested change

	def install
	generate_completions_from_executable(bin/"btp", "--autocomplete=init:", shells: [:bash, :zsh, :pwsh])
	end
	generate_completions_from_executable "btp", "--autocomplete=init:"

This is the required format for a Cask.

[paulchen5]

Wait do I also have to omit the shells here?

[bevanjkay]

Well I think I was mixing up casks here with formulas. Is there a reason why the cli tool is being counted as a cask?

Closed-source tools are always Casks, as they cannot be built from source as a Formula.