fix: complete Python 3.9 type annotation compatibility fixes

Fix remaining Python 3.9 incompatible type annotations throughout the
leann-core package that were causing test failures in CI. The union operator
(|) syntax for type hints was introduced in Python 3.10 and causes
"TypeError: unsupported operand type(s) for |" errors in Python 3.9.

Changes:
- Convert dict[str, Any] | None to Optional[dict[str, Any]]
- Convert int | None to Optional[int]
- Convert subprocess.Popen | None to Optional[subprocess.Popen]
- Convert LeannBackendFactoryInterface | None to Optional[LeannBackendFactoryInterface]
- Add missing Optional imports to all affected files

This resolves all test failures related to type annotation syntax and ensures
compatibility with Python 3.9 as specified in pyproject.toml.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

packages/leann-core/src/leann/api.py

@@ -10,7 +10,7 @@ import time
 import warnings
 from dataclasses import dataclass, field
 from pathlib import Path
-from typing import Any, Literal
+from typing import Any, Literal, Optional
 
 import numpy as np
 
@@ -33,7 +33,7 @@ def compute_embeddings(
     model_name: str,
     mode: str = "sentence-transformers",
     use_server: bool = True,
-    port: int | None = None,
+    port: Optional[int] = None,
     is_build=False,
 ) -> np.ndarray:
     """
@@ -157,12 +157,12 @@ class LeannBuilder:
         self,
         backend_name: str,
         embedding_model: str = "facebook/contriever",
-        dimensions: int | None = None,
+        dimensions: Optional[int] = None,
         embedding_mode: str = "sentence-transformers",
         **backend_kwargs,
     ):
         self.backend_name = backend_name
-        backend_factory: LeannBackendFactoryInterface | None = BACKEND_REGISTRY.get(backend_name)
+        backend_factory: Optional[LeannBackendFactoryInterface] = BACKEND_REGISTRY.get(backend_name)
         if backend_factory is None:
             raise ValueError(f"Backend '{backend_name}' not found or not registered.")
         self.backend_factory = backend_factory
@@ -242,7 +242,7 @@ class LeannBuilder:
         self.backend_kwargs = backend_kwargs
         self.chunks: list[dict[str, Any]] = []
 
-    def add_text(self, text: str, metadata: dict[str, Any] | None = None):
+    def add_text(self, text: str, metadata: Optional[dict[str, Any]] = None):
         if metadata is None:
             metadata = {}
         passage_id = metadata.get("id", str(len(self.chunks)))
@@ -592,7 +592,7 @@ class LeannChat:
     def __init__(
         self,
         index_path: str,
-        llm_config: dict[str, Any] | None = None,
+        llm_config: Optional[dict[str, Any]] = None,
         enable_warmup: bool = False,
         **kwargs,
     ):
@@ -608,7 +608,7 @@ class LeannChat:
         prune_ratio: float = 0.0,
         recompute_embeddings: bool = True,
         pruning_strategy: Literal["global", "local", "proportional"] = "global",
-        llm_kwargs: dict[str, Any] | None = None,
+        llm_kwargs: Optional[dict[str, Any]] = None,
         expected_zmq_port: int = 5557,
         **search_kwargs,
     ):

packages/leann-core/src/leann/chat.py

@@ -761,7 +761,7 @@ class SimulatedChat(LLMInterface):
         return "This is a simulated answer from the LLM based on the retrieved context."
 
 
-def get_llm(llm_config: dict[str, Any] | None = None) -> LLMInterface:
+def get_llm(llm_config: Optional[dict[str, Any]] = None) -> LLMInterface:
     """
     Factory function to get an LLM interface based on configuration.
 

packages/leann-core/src/leann/embedding_server_manager.py

@@ -6,6 +6,7 @@ import subprocess
 import sys
 import time
 from pathlib import Path
+from typing import Optional
 
 import psutil
 
@@ -182,8 +183,8 @@ class EmbeddingServerManager:
                                        e.g., "leann_backend_diskann.embedding_server"
         """
         self.backend_module_name = backend_module_name
-        self.server_process: subprocess.Popen | None = None
+        self.server_process: Optional[subprocess.Popen] = None
-        self.server_port: int | None = None
+        self.server_port: Optional[int] = None
         self._atexit_registered = False
 
     def start_server(

packages/leann-core/src/leann/searcher_base.py

@@ -1,7 +1,7 @@
 import json
 from abc import ABC, abstractmethod
 from pathlib import Path
-from typing import Any, Literal
+from typing import Any, Literal, Optional
 
 import numpy as np
 
@@ -169,7 +169,7 @@ class BaseSearcher(LeannBackendSearcherInterface, ABC):
         prune_ratio: float = 0.0,
         recompute_embeddings: bool = False,
         pruning_strategy: Literal["global", "local", "proportional"] = "global",
-        zmq_port: int | None = None,
+        zmq_port: Optional[int] = None,
         **kwargs,
     ) -> dict[str, Any]:
         """