chore(package): migrate to src/ layout (#3920)

Migrates package structure to src/ layout following Python packaging
best practices.

All code moved from `llama_stack/` to `src/llama_stack/`. Public API
unchanged - imports remain `import llama_stack.*`.

Updated build configs, pre-commit hooks, scripts, and GitHub workflows
accordingly. All hooks pass, package builds cleanly.

**Developer note**: Reinstall after pulling: `pip install -e .`

llama_stack/ui/components/responses/items/message-item.tsx

@@ -1,41 +0,0 @@
-import { MessageBlock } from "@/components/chat-playground/message-components";
-import { MessageItem } from "../utils/item-types";
-
-interface MessageItemProps {
-  item: MessageItem;
-  index: number;
-  keyPrefix: string;
-  defaultRole?: string;
-}
-
-export function MessageItemComponent({
-  item,
-  index,
-  keyPrefix,
-  defaultRole = "unknown",
-}: MessageItemProps) {
-  let content = "";
-
-  if (typeof item.content === "string") {
-    content = item.content;
-  } else if (Array.isArray(item.content)) {
-    content = item.content
-      .map(c => {
-        return c.type === "input_text" || c.type === "output_text"
-          ? c.text
-          : JSON.stringify(c);
-      })
-      .join(" ");
-  }
-
-  const role = item.role || defaultRole;
-  const label = role.charAt(0).toUpperCase() + role.slice(1);
-
-  return (
-    <MessageBlock
-      key={`${keyPrefix}-${index}`}
-      label={label}
-      content={content}
-    />
-  );
-}