fix(utils): Fix optional chaining error handling in inject_session_state

The previous implementation incorrectly treated entire paths as optional
if any segment contained '?', violating per-segment optional chaining
semantics. For example, {user.profile?} with missing 'user' would
incorrectly return empty string instead of raising an error.

Root cause: The catch block checked 'if '?' in full_path' to decide
whether to suppress KeyError. This was wrong because _get_nested_value()
already handles optional segments correctly by returning None for
missing optional parts. Any KeyError that propagates means a required
segment was missing and should be fatal.

Fix: Removed the conditional suppression logic. Now all KeyErrors from
_get_nested_value() are properly re-raised, maintaining correct
per-segment optional chaining semantics.

Examples:
- {user?.profile} with user missing → empty string (correct: user? is optional)
- {user.profile?} with user missing → KeyError (correct: user is required)
- {user?.profile?.role} with user missing → empty string (correct: chain stops at user?)

Added test case to verify required parent with optional child raises error.

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

Co-Authored-By: Claude <[email protected]>

Author: Jainish-S

src/google/adk/utils/instructions_utils.py

@@ -156,15 +156,8 @@ async def _replace_match(match) -> str:
         if value is None:
           return ''
         return str(value)
-      except KeyError:
-        if '?' in full_path:
-          logger.debug(
-              'Context variable %s not found (safe navigation), replacing with'
-              ' empty string',
-              full_path,
-          )
-          return ''
-        raise KeyError(f'Context variable not found: `{full_path}`.')
+      except KeyError as e:
+        raise KeyError(f'Context variable not found: `{full_path}`.') from e
 
   return await _async_sub(r'{+[^{}]*}+', _replace_match, template)
 

tests/unittests/utils/test_instructions_utils.py

@@ -391,6 +391,24 @@ async def test_inject_session_state_with_missing_nested_key_raises_error():
     )
 
 
+@pytest.mark.asyncio
+async def test_inject_session_state_with_required_parent_missing_raises_error():
+  """Test that {user.profile?} raises error when 'user' (required) is missing.
+
+  This verifies that optional chaining is per-segment, not for the whole path.
+  Even though 'profile?' is optional, 'user' is required and should raise error.
+  """
+  instruction_template = "Value: {user.profile?}"
+  invocation_context = await _create_test_readonly_context(state={})
+
+  with pytest.raises(
+      KeyError, match="Context variable not found: `user.profile\\?`"
+  ):
+    await instructions_utils.inject_session_state(
+        instruction_template, invocation_context
+    )
+
+
 @pytest.mark.asyncio
 async def test_inject_session_state_with_nested_and_prefixed_state():
   instruction_template = "User: {app:user.name} Temp: {temp:session.id}"