review: defer forward-time URL re-validation to follow-up (#2316)

Self-review found the original draft of this PR added forward-time
validateAgentURL() as defense-in-depth — paranoia layer on top of the
existing register-time gate. The validator unconditionally blocks
loopback (127.0.0.1/8), which makes httptest-based proxy tests
impossible without an env-var hatch I'd rather not add to a security-
critical path on first pass.

Trust note kept inline pointing at the upstream gate + tracking issue
so the gap is explicit, not invisible.

Refs #2312.

workspace-server/internal/handlers/chat_files.go

@@ -177,6 +177,10 @@ func (h *ChatFilesHandler) Upload(c *gin.Context) {
 		c.JSON(http.StatusServiceUnavailable, gin.H{"error": "workspace url not registered yet"})
 		return
 	}
+	// Trust note: workspaces.url passes validateAgentURL at /registry/
+	// register write time, blocking SSRF-shaped URLs. We rely on that
+	// upstream gate rather than re-validating here. Tracked at #2316
+	// for follow-up: forward-time re-validation as defense-in-depth.
 
 	secret, err := wsauth.ReadPlatformInboundSecret(ctx, db.DB, workspaceID)
 	if err != nil {