feat!: adopt Python 3.15 and support BattlesnakeArena

- Require Python 3.15 in project metadata, runtime pin, and Docker stages.
- Use lazy imports for database backends and type-only dependencies.
- Defer benchmark dependencies until their code paths are used.
- Rely on deferred annotations and use Path.info for file discovery.
- Accept BattlesnakeArena alongside BattlesnakeEngine on game endpoints.
- Advance quart_common for Python 3.15 and multi-agent validation.
- Add pytest development dependencies and refresh the uv lockfile.
- Test lazy backend loading, import order, and fresh file discovery.
- Document the runtime requirements and locked uv workflow.

BREAKING CHANGE: Python 3.15 or newer is required; earlier runtimes cannot parse the lazy import syntax.

server/dataset/Dataset.py

@@ -1,7 +1,4 @@
-from typing import TYPE_CHECKING
-
-if TYPE_CHECKING:
-  from server.GameBoard import GameBoard
+lazy from server.GameBoard import GameBoard
 
 class Dataset:
   VALID_MOVES = {"up", "down", "left", "right"}

server/dataset/DatasetIO.py

@@ -16,7 +16,7 @@ class DatasetIO:
 
     for item in input_files:
       path = Path(item)
-      if path.is_dir():
+      if path.info.is_dir():
         for pattern in patterns:
           for file_path in sorted(path.rglob(pattern)):
             key = str(file_path.resolve())
@@ -29,7 +29,7 @@ class DatasetIO:
       if any(ch in item for ch in "*?[]"):
         for match in sorted(glob.glob(item)):
           file_path = Path(match)
-          if not file_path.is_file():
+          if not file_path.info.is_file():
             continue
           key = str(file_path.resolve())
           if key in seen:
@@ -38,7 +38,7 @@ class DatasetIO:
           resolved.append(file_path)
         continue
 
-      if path.is_file():
+      if path.info.is_file():
         key = str(path.resolve())
         if key in seen:
           continue