fix(mcp): name raw argument bytes when brain_edit refuses an empty old_str

hyperguild#115: a session reported "old_str is empty" with old_str that
was never empty on their end. Server-side round-trip re-verified clean
(fresh brain_edit call, real content, succeeds) — the code path itself
isn't broken, but its error left no way to tell "client sent empty" apart
from "something between the client and here lost the field" without a
second investigation. The refusal now names the raw argument byte count.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ingestion/internal/mcp/tools_notes_test.go

@@ -70,6 +70,29 @@ func TestBrainReadAndEdit_RoundTripThroughJSONRPC(t *testing.T) {
 		"only the matched bytes may change")
 }
 
+func TestBrainEdit_EmptyOldStrErrorNamesRawArgumentBytes(t *testing.T) {
+	// hyperguild#115: a session reported "old_str is empty" with an old_str
+	// that, on their end, was never empty. The server-side path was
+	// unreachable to reproduce (verified separately: a fresh brain_edit call
+	// with real content round-trips cleanly), which points at something
+	// between a specific client and this call losing the field. The fix this
+	// issue actually asked for isn't a behavior change — old_str=="" must
+	// still refuse — it's that the refusal should say how many raw argument
+	// bytes it received, so "client sent empty" is distinguishable from
+	// "server received empty after some hop lost it" without a second
+	// session's worth of guessing.
+	dir := newBrainRepo(t)
+	srv := mcp.NewServer(dir, nil, nil, nil)
+
+	resp := call(t, srv, "brain_edit", map[string]any{
+		"path": "wiki/homelab/facts/n.md", "old_str": "", "new_str": "BETA",
+	})
+	errMsg, ok := resp["error"].(map[string]any)["message"].(string)
+	require.True(t, ok, "%v", resp)
+	assert.Contains(t, errMsg, "old_str is empty")
+	assert.Contains(t, errMsg, "raw arguments were", "%v", errMsg)
+}
+
 func TestBrainEdit_RefusesGovernanceFilesFromTheModelFacingTool(t *testing.T) {
 	dir := newBrainRepo(t)
 	srv := mcp.NewServer(dir, nil, nil, nil)