fix(completion): recover from panics in the application completer

[maxlandon]

Problem

readline calls the application-provided Completer on nearly every keystroke — both explicit menu completion and as-you-type autocomplete funnel through commandCompletion. If that user code panics (a bad completer, or transient command-tree state), the panic propagated out of the completion path and crashed the whole interactive shell.

Fix

Recover around the completer invocation in commandCompletion and surface the panic value as a completion message, so a single faulty completion degrades into a visible completion error: … instead of taking down the process. Both completion paths share this one adapter, so the single choke point covers them all.

func (rl *Shell) commandCompletion() (values completion.Values) {
    ...
    defer func() {
        if r := recover(); r != nil {
            msg := CompleteMessage("completion error: %v", r)
            values = msg.convert()
        }
    }()
    ...
}

Why here and not downstream

This is the readline-side fix for reeflective/console#81, which worked around the crash by recovering inside console's own completer. Fixing it in readline protects every caller — no downstream project needs to wrap its completer in a recover(). Once this lands, console #81 can be dropped (or trimmed to just its cobra command-tree reset, which is console-specific).

Testing

• New TestCommandCompletionRecoversFromPanic (panicking completer → recovered, message surfaced) and TestCommandCompletionNilCompleter (clean no-op).
• go build, go vet, and go test ./... all pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 39.33%. Comparing base (be2926f) to head (7ee6a10).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
+ Coverage   39.04%   39.33%   +0.28%     
==========================================
  Files          60       60              
  Lines        9606     9610       +4     
==========================================
+ Hits         3751     3780      +29     
+ Misses       5767     5742      -25     
  Partials       88       88              

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