fix: prevent action execution during shell completion after double dash

[toller892]

Description

When a shell autocomplete script appends --generate-bash-completion to a command line that already contains --, the completion flag is passed through to the action as a positional argument, causing the action to execute during a completion attempt.

This is particularly dangerous because tab-completion could trigger side effects (e.g., file creation, network calls) without the user pressing Enter.

Fixes #1993

Root Cause

PR #1938 added a check in checkShellCompleteFlag to disable shell completion when -- is present (correct behavior — after --, only positional args should be accepted). However, when this check fires, the function returns the unmodified arguments including --generate-bash-completion. The app then runs normally with --generate-bash-completion as a positional argument, executing the action.

Changes

help.go — checkShellCompleteFlag

• Always strip --generate-bash-completion from arguments when it's detected (regardless of -- presence), so it never leaks into the action's positional arguments
• Added a new return value completionDetected bool to indicate whether the completion flag was present
• Return (false, true, stripped) when -- is present (completion disabled, but flag was detected and stripped)
• Return (true, true, stripped) in the normal case (completion enabled, flag stripped)

app.go — RunContext

• Added early return (nil) when completionDetected && !shellComplete — the completion flag was present but shell completion is disabled (due to --). This prevents the action from executing during a completion attempt.

help_test.go

• Updated Test_checkShellCompleteFlag to verify the new completionDetected return value
• Updated the "arguments include double dash" test case: expected args changed from ["--", "foo", "--generate-bash-completion"] to ["--", "foo"]

Behavior Change

Scenario	Before	After
app --<TAB>	Action executes with --generate-bash-completion as arg	Returns nil, action not executed
app -- foo <TAB>	Action executes with --generate-bash-completion as arg	Returns nil, action not executed
app foo <TAB>	Completion works normally	No change
app <TAB>	Completion works normally	No change