fix: don't leak internal error detail in GET /v0/servers 500 response (#1338)

## Summary
Fixes the information-disclosure regression introduced by #1335 (item 1
of #1337).

`ListServersError` passed the raw `err` into
`huma.Error500InternalServerError(...)`. huma serializes extra error
args into the response body's `errors` array (`ErrorModel.Errors`,
`json:"errors,omitempty"`), so on the **public, unauthenticated** `GET
/v0/servers` endpoint a genuine DB failure echoed the internal wrapped
error from `internal/database/postgres.go` (`error iterating rows: <pgx
error>`) back to the client. pgx/pgconn errors can carry SQLSTATE codes,
constraint/table/column names, and internal messages (CWE-209).

## Change
- Drop `err` from the 500 call so only the generic message is returned;
keep `log.Printf` for the server-side detail — matching the sibling
handlers (`servers.go:190/220`, `edit.go`, `status.go`).
- Add a regression test
(`TestListServersError_realFailureDoesNotLeakDetail`) that marshals the
500 error and asserts the internal detail is absent from the
client-visible body. Existing tests only asserted status codes, which is
why the leak slipped through.

The 499 client-cancelled path is unchanged (the client has already
disconnected, so nothing is leaked there).

## Test plan
- [x] `go test ./internal/api/handlers/v0/... -run ListServersError`
(3/3 pass)
- [x] `go build ./...`, `go vet`, `golangci-lint` (no new findings in
changed files)

## Note
This leaves the non-security items in #1337 (correcting the `superfluous
WriteHeader` claim, `statusClientClosed` const reuse, broader
cancellation-guard scope) for separate follow-up. Closing #1337 here
since the only security-impacting item is addressed; reopen/split if
you'd prefer to track the nits independently.

Closes #1337

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/api/handlers/v0/list_errors.go

@@ -17,5 +17,8 @@ func ListServersError(ctx context.Context, err error) error {
 		return huma.NewError(499, "Client closed request", err)
 	}
 	log.Printf("list servers failed: %v", err)
-	return huma.Error500InternalServerError("Failed to get registry list", err)
+	// Do not pass err here: huma serializes extra error args into the response
+	// body, which would leak internal (e.g. pgx) error detail to clients. Log it
+	// server-side only, like the sibling handlers in servers.go.
+	return huma.Error500InternalServerError("Failed to get registry list")
 }

internal/api/handlers/v0/list_errors_test.go

@@ -2,6 +2,7 @@ package v0_test
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"testing"
 
@@ -29,3 +30,16 @@ func TestListServersError_realFailure(t *testing.T) {
 	require.True(t, errors.As(err, &se))
 	assert.Equal(t, 500, se.GetStatus())
 }
+
+// The 500 response body must not echo the internal error back to the client,
+// since it can carry DB/driver detail (CWE-209).
+func TestListServersError_realFailureDoesNotLeakDetail(t *testing.T) {
+	const internal = "database unavailable: dsn=postgres://user:pw@internal-host/db"
+	err := v0.ListServersError(context.Background(), errors.New(internal))
+	require.Error(t, err)
+
+	body, marshalErr := json.Marshal(err)
+	require.NoError(t, marshalErr)
+	assert.NotContains(t, string(body), "database unavailable")
+	assert.NotContains(t, string(body), "internal-host")
+}