refactor: reorganize client transport and extension internals

- Split client, native, remote, serve, markdown, and SDK internals into focused packages with direct imports.
- Move local and remote transport framing/protocol helpers behind clearer module boundaries.
- Break up the extension injected DOM logic into a separate content dispatch bundle and dedicated content modules.
- Add explicit client handling for passive remote discovery without noisy PQ warnings.
- Keep behavior covered with updated unit, integration, and extension tests.

browser_cli/client/messages.py

@@ -0,0 +1,45 @@
+"""Command message/response helpers shared by sync and async clients."""
+import json
+import os
+import uuid
+from typing import Any
+
+from browser_cli import transport
+from browser_cli.endpoints import _normalize_endpoint
+from browser_cli.errors import BrowserNotConnected
+
+def base_message(command: str, args: dict | None) -> dict:
+  return {"id": str(uuid.uuid4()), "command": command, "args": args or {}}
+
+def requested_target(profile: str | None, remote: str | None) -> tuple[str | None, str | None]:
+  requested_profile = profile or os.environ.get("BROWSER_CLI_PROFILE")
+  remote_endpoint = remote or os.environ.get("BROWSER_CLI_REMOTE")
+  return requested_profile, _normalize_endpoint(remote_endpoint) if remote_endpoint else None
+
+def encode_payload(msg: dict) -> bytes:
+  return json.dumps(msg).encode("utf-8")
+
+def decode_response(response: bytes | None) -> Any:
+  if response is None:
+    raise ConnectionError("Connection closed before full response received")
+  result = transport.decode_response(response)
+  if not result.get("success", True):
+    raise RuntimeError(result.get("error", "unknown error from browser"))
+  return result.get("data")
+
+def local_connection_error(profile: str | None) -> BrowserNotConnected:
+  profile_hint = f" (profile: {profile})" if profile else ""
+  return BrowserNotConnected(
+    f"Cannot connect to browser{profile_hint}.\n"
+    "Make sure:\n"
+    "  1. The browser-cli extension is installed and enabled\n"
+    "  2. The native host is registered: uv run browser-cli install <browser>\n"
+    "  3. Your browser is running\n"
+    "  Tip: use BROWSER_CLI_PROFILE=<name> to select a specific profile"
+  )
+
+def remote_connection_error(remote_endpoint: str) -> BrowserNotConnected:
+  return BrowserNotConnected(
+    f"Cannot connect to remote browser at {remote_endpoint}.\n"
+    "Make sure browser-cli serve is running on the remote host."
+  )