fix(agent): coerce non-object tool-call arguments instead of crashing (#1370)

A native function/tool call whose `arguments` field is valid JSON but not an
object — a bare array like ["ls -la"], or a string/number/bool/null — parsed
fine in function_call_to_tool_block and then every branch called args.get(...),
raising AttributeError ('list'/'str' object has no attribute 'get'). That
propagated out of the streamed agent loop (no surrounding try/except at the
call site in stream_agent_loop) and aborted the user's entire turn. Weaker and
local models routinely emit malformed args like this.

Coerce non-dict parsed arguments to {} (mirrors the existing empty-arguments
behavior), so the tool runs with empty args instead of killing the stream.

Adds tests/test_function_call_non_object_args.py covering array/string/number/
bool/null arguments — they fail before this change and pass after.

tests/test_function_call_non_object_args.py

@@ -0,0 +1,37 @@
+import sys
+from unittest.mock import MagicMock
+
+# Clean up any mocks from previous tests to ensure we load real modules
+for mod in ['src.agent_tools', 'src.tool_parsing', 'src.tool_schemas', 'src.tool_execution']:
+    sys.modules.pop(mod, None)
+
+# Mock heavy database/model dependencies before importing
+for mod in [
+    'sqlalchemy', 'sqlalchemy.orm', 'sqlalchemy.ext', 'sqlalchemy.ext.declarative',
+    'sqlalchemy.ext.hybrid', 'sqlalchemy.sql', 'sqlalchemy.sql.expression',
+    'src.database', 'core.models', 'core.database', 'core.auth'
+]:
+    if mod not in sys.modules:
+        sys.modules[mod] = MagicMock()
+
+import pytest
+import src.agent_tools  # noqa: F401
+from src.tool_schemas import function_call_to_tool_block
+
+
+@pytest.mark.parametrize("arguments", [
+    '["ls -la"]',   # JSON array
+    '"ls -la"',     # bare JSON string
+    '42',            # JSON number
+    'true',          # JSON bool
+    'null',          # JSON null
+])
+def test_non_object_arguments_do_not_crash(arguments):
+    """A native function call whose arguments are valid JSON but not an object
+    must not raise (it used to throw AttributeError: 'list' object has no
+    attribute 'get', aborting the entire agent stream)."""
+    block = function_call_to_tool_block("bash", arguments)
+    # Coerced to empty args -> empty bash command, but importantly NO crash.
+    assert block is not None
+    assert block.tool_type == "bash"
+    assert block.content == ""