fix: concatenate list values in deep_merge_dicts during parallel tool call merge

[enjoykumawat]

Summary

• Bug: When multiple tool calls run in parallel and each writes to the same state_delta key containing a list value, deep_merge_dicts silently drops all but the last value because lists hit the else branch and get overwritten.
• Fix: Add a list-type check in deep_merge_dicts so list values are concatenated (d1[key] + value) instead of overwritten, preserving all entries from parallel function responses.
• Tests: Added 5 unit tests covering list concatenation, scalar overwrite, nested dict merge, mixed-type handling, and an integration test for merge_parallel_function_response_events with state_delta lists.

Reproduction

# Tool A's delta: {"state_delta": {"items": ["a"]}}
# Tool B's delta: {"state_delta": {"items": ["b"]}}
# Before fix: {"state_delta": {"items": ["b"]}} — item "a" is lost
# After fix:  {"state_delta": {"items": ["a", "b"]}} — both preserved

Test plan

• All 5 new unit tests pass (test_deep_merge_dicts_* and test_merge_parallel_function_response_events_merges_state_delta_lists)
• All 32 existing tests in test_functions_simple.py pass (0 regressions)
• Manual: create parallel tool calls that accumulate list state and verify no data loss

Fixes #5190

[wuliang229]

Please move the tests to test_functions_parallel.py

[enjoykumawat]

Done! Moved all the deep_merge_dicts and state_delta list tests to test_functions_parallel.py in commit 4b3db55. Also merged latest main to pick up any upstream changes. CI is re-running now.