Python: fix: prevent inner_exception from being lost in AgentFrameworkException (#5167)

* fix: prevent inner_exception from being lost in AgentFrameworkException

The __init__ method unconditionally called super().__init__() after
the conditional call with inner_exception, effectively overwriting the
exception args and losing the inner_exception reference.

Add else branch so super().__init__() is only called once with the
correct arguments.

Fixes #5155

Signed-off-by: bahtya <bahtyar153@qq.com>

* test: add explicit tests for AgentFrameworkException inner_exception handling

- test_exception_with_inner_exception: verifies args include inner exception
- test_exception_without_inner_exception: verifies args only contain message
- test_exception_inner_exception_none_explicit: verifies explicit None

Covers both branches of the if/else in __init__.

* fix: export AgentFrameworkException from package

Bahtya

---------

Signed-off-by: bahtya <bahtyar153@qq.com>

python/packages/core/tests/core/test_exceptions.py

@@ -0,0 +1,29 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+"""Tests for AgentFrameworkException inner_exception handling."""
+
+import pytest
+
+from agent_framework import AgentFrameworkException
+
+
+def test_exception_with_inner_exception():
+    """When inner_exception is provided, it should be set as the second arg."""
+    inner = ValueError("inner error")
+    exc = AgentFrameworkException("test message", inner_exception=inner)
+    assert exc.args[0] == "test message"
+    assert exc.args[1] is inner
+
+
+def test_exception_without_inner_exception():
+    """When inner_exception is None, args should only contain the message."""
+    exc = AgentFrameworkException("test message")
+    assert exc.args == ("test message",)
+    assert len(exc.args) == 1
+
+
+def test_exception_inner_exception_none_explicit():
+    """When inner_exception is explicitly None, args should only contain the message."""
+    exc = AgentFrameworkException("test message", inner_exception=None)
+    assert exc.args == ("test message",)
+    assert len(exc.args) == 1