Autocomplete argument reconstruction does not join tokens after a trailing colon

Summary

rebuildColonSeparatedArgs documents that it rejoins shell-completion arguments when either the next token is ":" or the current token already ends with ":". The implementation enters the loop for the second case but immediately breaks instead of appending the following token.

As a result, a tokenization shape such as:

[]string{"config:", "get"}

remains two arguments instead of being reconstructed as:

[]string{"config:get"}

This can make colon-named commands fail to resolve correctly when a shell presents the word-break boundary after the colon rather than returning the colon as its own token.

Reproduction

On current main (d082a010f7c6cacf407d8a1581446a7857f9f1bb):

got := rebuildColonSeparatedArgs([]string{"config:", "get"})
// current: []string{"config:", "get"}
// expected: []string{"config:get"}

The function's existing comment explicitly describes the intended rule:

Keep joining while the next element is ":" or the current element ends with ":"

but the current loop body contains:

if args[i+1] == ":" {
    // append colon and possibly the following element
    ...
} else {
    break
}

So the strings.HasSuffix(current, ":") half of the loop condition cannot actually join anything unless the next token is also a standalone colon.

Expected behavior

Both common shell tokenization shapes should reconstruct to the same logical command token:

[]string{"config", ":", "get"} -> []string{"config:get"}
[]string{"config:", "get"}      -> []string{"config:get"}

Consecutive colon boundaries should also remain intact, e.g. {"a", ":", ":", "b"} -> {"a::b"}.

Suggested fix

While the next token is a standalone colon or the accumulated token ends in a colon, append the next token directly and advance. This removes the contradictory inner branch and handles both boundary shapes uniformly.

Add focused table-driven coverage for standalone-colon, trailing-colon, repeated-colon, and ordinary no-colon inputs.

Impact

This is shell-completion correctness. Depending on how the invoking shell splits a colon command at the cursor boundary, the completion helper can reconstruct a different argv shape and therefore miss the command or return irrelevant completions.

Start at the rebuildColonSeparatedArgs function and inspect its existing callers and tests. Add focused table-driven coverage for standalone-colon, trailing-colon, repeated-colon, and ordinary no-colon inputs, then run the relevant Go tests and confirm both tokenization shapes reconstruct the same command token.