Python: expose BamlAbortError (#2674)

## Summary
Align Python abort error behavior with docs and TS:
- Expose `BamlAbortError` in the Python typing stub
- Raise native `BamlAbortError` on early‑abort (pre‑runtime) instead of
a generic Exception
- Add tests to validate import/isinstance and early‑abort type

## Motivation / Context
- Docs: `fern/03-reference/baml_client/errors/overview.mdx` describe
error types and behavior.
- In Python:
  - Runtime aborts already surface as native `BamlAbortError` via PyO3
- Early‑abort (already‑aborted controller) raised a generic
`Exception("BamlAbortError: ...")`
  - `baml_py.pyi` didn’t declare `BamlAbortError`
- This PR makes Python match the documented behavior and enables type
checkers to see `BamlAbortError`.

## Changes
- engine/language_client_python/python_src/baml_py/baml_py.pyi
  - Add `class BamlAbortError(BamlError): ...`
- engine/generators/languages/python/src/_templates/runtime.py.j2
- Early‑abort now raises `baml_py.baml_py.BamlAbortError("Operation was
aborted")`
- integ-tests/python/tests/test_abort_handlers.py
  - Update `test_early_abort` to expect `BamlAbortError`
  - Add `test_baml_abort_error_import_and_instanceof` sanity check

Not included: regenerated client artifacts, to keep diff focused.
Regeneration picks up the template change.

## Testing
Targeted tests (run without provider keys):
```bash
cd integ-tests/python
uv sync
uv run maturin develop --uv --manifest-path ../../engine/language_client_python/Cargo.toml
uv run baml-cli generate --from ../baml_src
uv run pytest -q -k 'early_abort or baml_abort_error_import_and_instanceof'
# Result: both tests passed locally
```

Full Python suite (requires provider env vars in `integ-tests/.env`):
```bash
cd integ-tests/python
uv sync
uv run maturin develop --uv --manifest-path ../../engine/language_client_python/Cargo.toml
uv run baml-cli generate --from ../baml_src
dotenv -e ../.env -- uv run pytest -q
```

## Backwards Compatibility
- Early‑abort Exception type changes from a generic `Exception` to
`BamlAbortError` (more specific); downstream code that caught generic
errors continues to work. Typed checks (`isinstance(e, BamlAbortError)`)
now work consistently for both early and in‑flight aborts.

Author: hellovai

engine/generators/languages/python/src/_templates/runtime.py.j2

@@ -124,7 +124,7 @@ class {{ call_manager_name }}:
 
         # Check if already aborted
         if resolved_options.abort_controller is not None and resolved_options.abort_controller.aborted:
-            raise Exception("BamlAbortError: Operation was aborted")
+            raise baml_py.baml_py.BamlAbortError("Operation was aborted")
 
         return await __runtime__.call_function(
             function_name,
@@ -154,7 +154,7 @@ class {{ call_manager_name }}:
 
         # Check if already aborted
         if resolved_options.abort_controller is not None and resolved_options.abort_controller.aborted:
-            raise Exception("BamlAbortError: Operation was aborted")
+            raise baml_py.baml_py.BamlAbortError("Operation was aborted")
 
         ctx = __ctx__manager__.get()
         return __runtime__.call_function_sync(

engine/language_client_python/python_src/baml_py/baml_py.pyi

@@ -561,3 +561,8 @@ class BamlClientHttpError(BamlClientError):
     """Raised for HTTP-related client errors."""
 
     ...
+
+class BamlAbortError(BamlError):
+    """Raised when a BAML operation is cancelled (abort)."""
+
+    ...

integ-tests/python-v1/baml_client/runtime.py

@@ -3,6 +3,7 @@
 import time
 from baml_client import b
 from baml_py import AbortController
+from baml_py.errors import BamlAbortError
 
 
 @pytest.mark.asyncio
@@ -170,12 +171,17 @@ async def test_early_abort():
     abort_controller = AbortController()
     abort_controller.abort()
 
-    with pytest.raises(Exception) as exc_info:
+    with pytest.raises(BamlAbortError):
         await b.ExtractName(
             text="John Doe", baml_options={"abort_controller": abort_controller}
         )
 
-    assert "abort" in str(exc_info.value).lower()
+
+def test_baml_abort_error_import_and_instanceof():
+    try:
+        raise BamlAbortError("Operation was aborted")
+    except Exception as e:
+        assert isinstance(e, BamlAbortError)
 
 
 @pytest.mark.asyncio