Run Before hooks before shell completion

[ibobgunardi]

What type of PR is this?

• bug

What this PR does / why we need it:

• runs the resolved command chain's Before hooks before invoking shell completion
• keeps completion before flag/action validation, so malformed input can still reach custom completion
• adds a nested custom completion regression test for setup done in Before

Which issue(s) this PR fixes:

Fixes #2348

Testing

• go test ./... -count=1

Release Notes

Shell completion now runs command `Before` hooks before custom completion callbacks.

[dearchap]

@ibobgunardi Excellent work. Very nicely done. I am just wondering if having Before execute for shell completion needs to be behind a flag. I dont know what users want. Having Before triggered by default or not. @urfave/cli please chime in.

[abitrolly]

@dearchap I need a diagram with expanded explanation of steps like Before to remind myself what the workflow is. From the issue description it looks like there are two pathways - run as normal and run as completion, and while this feature fixes the last one, I am not sure what are typical use cases of Before in both pathways to evaluate the impact.

[ibobgunardi]

Here is the split I had in mind:

normal run:
  parse args -> resolve command chain -> run Before hooks -> validate/run action

completion run:
  parse enough to resolve command/flag -> run Before hooks -> call completion callback -> exit

The practical case is shared setup that both the action and completion need: loading config, initializing command-scoped state, or preparing a client/cache that the completion callback reads from. The completion path still exits before action execution and required-flag validation; this change just gives completion the same command setup step that a normal run gets.

The regression test uses a nested command where Before fills state that the completion callback reads, because that is the smallest way to show the missing setup without pulling in external state.