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/llm/test_helpers_test.go b/internal/llm/test_helpers_test.go
index b6553bf..0280af2 100644
--- a/internal/llm/test_helpers_test.go
+++ b/internal/llm/test_helpers_test.go
@@ -8,7 +8,9 @@ import (
 // TestMain registers all built-in providers before any test runs, mirroring
 // the explicit registration that happens in production binaries.
 func TestMain(m *testing.M) {
-	RegisterAllProviders()
+	if err := RegisterAllProviders(); err != nil {
+		panic(err)
+	}
 	os.Exit(m.Run())
 }