Fix/improve typing for get_codebase_session using default programming_language (#275)

# Motivation

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# Content

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# Testing

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Author: EdwardJXLi

src/codegen/sdk/codebase/factory/get_session.py

@@ -15,6 +15,22 @@
 from codegen.sdk.tree_sitter_parser import print_errors
 
 
+@overload
+def get_codebase_session(
+    tmpdir: str | os.PathLike[str],
+    programming_language: None = None,
+    files: dict[str, str] = {},
+    commit: bool = True,
+    sync_graph: bool = True,
+    verify_input: bool = True,
+    verify_output: bool = True,
+    repo_config: BaseRepoConfig | None = None,
+    feature_flags: GSFeatureFlags = TestFlags,
+    session_options: SessionOptions = SessionOptions(),
+    secrets: Secrets = Secrets(),
+) -> AbstractContextManager[PyCodebaseType]: ...
+
+
 @overload
 def get_codebase_session(
     tmpdir: str | os.PathLike[str],