fix(nodes): keep every key of an AND group in parentheses

[MohammadHijjawi97]

Summary

Node input expressions are resolved by BaseNode._parse_input_keys: parenthesised groups are evaluated first, and their keys are spliced back into the expression. The keys were joined with |, which turns an AND group into an OR, so required keys are silently dropped:

state = {"a": 1, "b": 2, "c": 3}
# input="a & (b & c)"  -> before: ['a', 'b']   after: ['a', 'b', 'c']
# input="(a & b) & c"  -> before: ['a']        after: ['a', 'b', 'c']
# input="(a & b) | c"  -> before: ['a']        after: ['a', 'b']

Changes

• Join the keys of an evaluated group with &, so they remain required together. Groups that resolve to a single key behave exactly as before, e.g. user_prompt & (relevant_chunks | parsed_doc | doc) used by the built-in graphs.
• The standalone utils.parse_state_keys.parse_expression helper duplicates this logic, so it gets the same one-line fix.
• Added tests/nodes/base_node_test.py and a case in tests/utils/parse_state_keys_test.py.

Testing

• pytest tests/nodes/base_node_test.py tests/utils/parse_state_keys_test.py: 4 of the new cases fail on pre/beta, and all 6 pass with the fix.
• pytest tests/graphs/abstract_graph_test.py: 24 passed. test_llm_missing_tokens fails identically on unmodified pre/beta and is unrelated.
• ruff check on the changed files; black --check / isort --check-only on the test files.