Keep the API key out of %#v for redacted LLM errors

The OpenAI adapter's redactedError had no GoString, so %#v printed its
fields, including the raw API key. Add GoString returning the redacted
text, matching the Anthropic adapter; redactedJoin inherits it.

Add TestRedactedErrorsFormatWithoutKey to both adapters, formatting
redactedError, redactedJoin and transportFailure with %v, %+v and %#v.
llm.ProviderError and llm.RateLimitedError keep no secret and are
already covered by TestTypedErrorsDoNotKeepSecret.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ai/llm/openai/openai.go

@@ -530,6 +530,9 @@ type redactedError struct {
 
 func (e *redactedError) Error() string { return e.text }
 
+// GoString keeps %#v from printing the fields, which hold the key.
+func (e *redactedError) GoString() string { return e.text }
+
 func (e *redactedError) Unwrap() error { return redact(errors.Unwrap(e.err), e.key) }
 
 func (e *redactedError) Is(target error) bool { return errors.Is(e.err, target) }