fix(knowledge): guard TreeKnowledge.find_path against malformed parent links

[saivedant169]

What

TreeKnowledge.find_path could raise KeyError or loop forever on malformed parent links.

Why

The walk-up loop followed parent_id without checking the target stayed in nodes:

• a parent_id pointing at a node not in the map raised KeyError
• a cycle in the parent links (a points to b, b points to a) looped forever

TreeNode.parent_id is UUID | None with no referential validator, and a Paper (a TreeKnowledge) is populated by Agent(output_type=Paper), so the parent links come from model output and carry no guarantee. Both cases are reachable from normal extraction.

Fix

Stop the walk when the chain leaves the node map or revisits a node, tracking visited ids to break cycles. The path still ends at the requested node and returns the best-effort partial path for a broken chain instead of raising or hanging. The happy path is unchanged.

I went with a partial path rather than [] on a broken chain so callers still get the resolvable ancestors. Happy to switch to [] if you prefer a stricter contract.

Tests

• test_find_path_dangling_parent: parent_id points outside the map, asserts no KeyError
• test_find_path_cyclic_parent: cyclic parent links, asserts it terminates

I confirmed both fail on the current code first (KeyError and an infinite loop) and pass with the fix.

python -m unittest discover -s tests/knowledge is green (66 tests). ruff check and ruff format --check pass on both files.