feat(debug): enable Python debug workflow and debug server integration

Adds an end-to-end Python debug workflow for the extension.
Includes a new prepare-debug.ps1 script and a VS Code task.
Extends the Python server and extension to coordinate a debug session and safe startup.

- Add prepare-debug.ps1 and a VS Code task to build the debug bundle
- Enable Python debug wiring in the server and tests
- Ensure a single stable Python debug session during startup

server/src/_debug_server.py

@@ -6,6 +6,7 @@ import os
 import pathlib
 import runpy
 import sys
+import time
 
 
 def update_sys_path(path_to_add: str) -> None:
@@ -14,10 +15,43 @@ def update_sys_path(path_to_add: str) -> None:
         sys.path.append(path_to_add)
 
 
-# Ensure debugger is loaded before we load anything else, to debug initialization.
-debugger_path = os.getenv("DEBUGPY_PATH", None)
-if debugger_path:
-    if debugger_path.endswith("debugpy"):
+def _debug_endpoint() -> tuple[str, int]:
+    host = os.getenv("NXPS_DEBUG_HOST", "127.0.0.1")
+    raw_port = os.getenv("NXPS_DEBUG_PORT", "5678")
+    try:
+        port = int(raw_port)
+    except ValueError as error:
+        raise RuntimeError(f"Invalid NXPS_DEBUG_PORT: {raw_port!r}") from error
+    if not 1 <= port <= 65535:
+        raise RuntimeError(f"NXPS_DEBUG_PORT is outside the valid range: {port}")
+    return host, port
+
+
+def _connect_debugger(debugpy, host: str, port: int, timeout: float = 15.0) -> None:
+    deadline = time.monotonic() + timeout
+    last_error: OSError | None = None
+    while time.monotonic() < deadline:
+        try:
+            debugpy.connect((host, port))
+            debugpy.wait_for_client()
+            return
+        except (ConnectionRefusedError, OSError) as error:
+            last_error = error
+            time.sleep(0.25)
+
+    raise RuntimeError(
+        f"Could not connect debugpy to {host}:{port} within {timeout:.0f} seconds"
+    ) from last_error
+
+
+def main() -> None:
+    # Ensure debugger is loaded before we load anything else, so server
+    # initialization and module-level feature registration can be debugged.
+    debugger_path = os.getenv("DEBUGPY_PATH")
+    if not debugger_path:
+        raise RuntimeError("DEBUGPY_PATH is missing in Python debug mode")
+
+    if pathlib.Path(debugger_path).name.casefold() == "debugpy":
         debugger_path = os.fspath(pathlib.Path(debugger_path).parent)
 
     update_sys_path(debugger_path)
@@ -25,25 +59,14 @@ if debugger_path:
     # pylint: disable=wrong-import-position,import-error
     import debugpy
 
-    # 5678 is the default port, If you need to change it update it here
-    # and in launch.json.
-    # Connecting requires the "Python debug server" listener (launch.json) to be
-    # up first. If it isn't (e.g. wrong launch config was used), don't crash the
-    # whole language server - just continue running without the debugger attached.
-    try:
-        debugpy.connect(5678)
-    except (ConnectionRefusedError, OSError) as exc:
-        print(
-            f"debugpy: could not connect to debug adapter on port 5678 "
-            f"({exc}); continuing without debugging.",
-            file=sys.stderr,
-        )
+    host, port = _debug_endpoint()
+    print(f"debugpy: waiting for VS Code at {host}:{port}", file=sys.stderr)
+    _connect_debugger(debugpy, host, port)
+    print("debugpy: VS Code attached; starting language server", file=sys.stderr)
 
-    # This will ensure that execution is paused as soon as the debugger
-    # connects to VS Code. If you don't want to pause here comment this
-    # line and set breakpoints as appropriate.
-    # debugpy.breakpoint()
+    server_path = os.fspath(pathlib.Path(__file__).parent / "lsp_server.py")
+    runpy.run_path(server_path, run_name="__main__")
 
-SERVER_PATH = os.fspath(pathlib.Path(__file__).parent / "lsp_server.py")
-# NOTE: Set breakpoint in `lsp_server.py` before continuing.
-runpy.run_path(SERVER_PATH, run_name="__main__")
+
+if __name__ == "__main__":
+    main()