fix(coderd/agentapi): trim whitespace from workspace agent metadata values (#23709)

- Trim leading/trailing whitespace from metadata `value` and `error`
fields before storage
- Trimming happens before length validation so whitespace-padded values
are handled correctly
- Add `TrimWhitespace` test covering spaces, tabs, newlines, and
preserved inner whitespace
- No backfill needed (unlogged table, stores only latest value)

> 🤖 Created by a Coder Agent, reviewed by me.

coderd/agentapi/metadata.go

@@ -3,6 +3,7 @@ package agentapi
 import (
 	"context"
 	"fmt"
+	"strings"
 	"time"
 
 	"github.com/google/uuid"
@@ -60,6 +61,8 @@ func (a *MetadataAPI) BatchUpdateMetadata(ctx context.Context, req *agentproto.B
 		}
 	)
 	for _, md := range req.Metadata {
+		md.Result.Value = strings.TrimSpace(md.Result.Value)
+		md.Result.Error = strings.TrimSpace(md.Result.Error)
 		metadataError := md.Result.Error
 
 		allKeysLen += len(md.Key)