fix(evaluator): strip PEP 604 union return annotations before upload (#167)

[Josiah Coad (josiahcoad)]

Fixes #167.

Problem

langsmith evaluator upload fails with a misleading error when the Python entrypoint has a PEP 604 union return annotation (-> dict | None):

HTTP 400: Code evaluator does not contain function with name perform_eval

The function is present. The evaluator service's function finder just doesn't handle union return annotations — a plain annotation (-> dict) or no annotation uploads fine. Confirmed with three files identical except the annotation: no-annotation ✅, -> dict ✅, -> dict | None ❌.

Fix

Strip the return-type annotation from the def line of the extracted/renamed Python function before upload (it's irrelevant to execution), preserving the params and any trailing comment:

def perform_eval(run) -> dict | None:        ->  def perform_eval(run):
def perform_eval(run, ex) -> dict | None:  # x ->  def perform_eval(run, ex):  # x
def perform_eval(run) -> dict:               ->  def perform_eval(run):
def perform_eval(run):                       ->  (unchanged)

This is a client-side mitigation; the underlying service-side function finder could also be made union-aware, but stripping the annotation is safe and unblocks users today.

Tests

• Added TestStripPythonReturnAnnotation_* (union, plain, params+comment, no-annotation-unchanged).
• go test ./internal/cmd/ passes; gofmt/go vet clean.
• Verified end-to-end against the live API: a -> dict | None evaluator (incl. a trailing comment) that previously returned HTTP 400 now uploads successfully with the patched binary.