Replace panics with returned errors in llm.RegisterProvider

RegisterProvider now returns an error for an empty name, a nil factory,
or a duplicate registration instead of panicking, making registration
composable and testable without recover(). RegisterAllProviders caches
the one-time registration error (sync.Once cannot return a value) and
returns it to every caller.

Updated all call sites: hexailsp, hexaicli and hexaiaction propagate the
error with context; test TestMains fail fast via panic. Added unit tests
covering empty name, nil factory, duplicate, success and idempotent
re-registration.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/internal/hexaiaction/cmdentry.go b/internal/hexaiaction/cmdentry.go
index d60f172..ee78307 100644
--- a/internal/hexaiaction/cmdentry.go
+++ b/internal/hexaiaction/cmdentry.go
@@ -25,7 +25,9 @@ type Options struct {
 // RunCommand is the CLI orchestrator used by cmd/hexai-tmux-action. It runs in tmux
 // split-pane mode by default, or child mode when -ui-child is set.
 func RunCommand(ctx context.Context, opts Options, stdin io.Reader, stdout, stderr io.Writer) error {
-	llm.RegisterAllProviders()
+	if err := llm.RegisterAllProviders(); err != nil {
+		return fmt.Errorf("failed to register LLM providers: %w", err)
+	}
 	if opts.UIChild {
 		return runChild(ctx, opts.Infile, opts.Outfile, stdout, stderr)
 	}