revert: replace fullwidth pipe ｜ with halfwidth | in DSML tool markup

PR #460 introduced fullwidth pipe characters (｜) in DSML tool call formatting
to improve parsing robustness, but models exposed to these fullwidth pipes in
system prompts exhibit significantly higher rates of tool output hallucinations.
Reverting to halfwidth pipes (|) drastically reduces tokenizer/perplexity-driven
hallucinations while retaining the existing confusable-hardening in the parser.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/httpapi/openai/file_inline_upload_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -41,8 +42,12 @@ func (m *inlineUploadDSStub) UploadFile(ctx context.Context, _ *auth.RequestAuth
 	if m.uploadErr != nil {
 		return nil, m.uploadErr
 	}
+	id := "file-inline-1"
+	if len(m.uploadCalls) > 1 {
+		id = "file-inline-" + fmt.Sprint(len(m.uploadCalls))
+	}
 	return &dsclient.UploadFileResult{
-		ID:       "file-inline-1",
+		ID:       id,
 		Filename: req.Filename,
 		Bytes:    int64(len(req.Data)),
 		Status:   "uploaded",