[lmeyerov]

Summary

• Fixes GFQL: runtime errors inside Call handlers are reported as "[invalid-node-reference] parameter validation failed" (E303), misdirecting callers #2196 by giving exceptions raised inside safelisted Call handlers a structured GFQLCallExecutionError / E306 (call-execution-error).
• Includes the handler name, requested engine, original exception type, and original exception as __cause__.
• Preserves E303/E105 parameter and reference validation errors and existing NotImplementedError engine-capability behavior.

Exact head: df9d7bf06
Base: master@0c71cc4bb8281ecc1f07de8657c4ec45c737a5c1

Evidence

python -m pytest -q \
  graphistry/tests/compute/gfql/test_alias_scoping_semantics.py::test_scalar_rebind_stays_an_error \
  graphistry/tests/compute/test_call_runtime_errors.py \
  graphistry/tests/layout/test_circle_entry_guards.py
# 17 passed, 6 skipped

python -m pytest -q \
  graphistry/tests/compute/test_call_operations.py \
  graphistry/tests/compute/test_call_schema_validation.py \
  graphistry/tests/compute/test_gfql_call_validation.py \
  graphistry/tests/compute/test_call_boundaries.py \
  graphistry/tests/compute/test_call_runtime_errors.py
# 134 passed

python -m ruff check graphistry/compute/exceptions.py \
  graphistry/compute/gfql/call/executor.py \
  graphistry/tests/compute/test_call_runtime_errors.py
# All checks passed

python ./bin/ci/ci_type_hygiene_guard.py
# Type-hygiene guard OK (4446 grandfathered finding(s), no growth)

python ./bin/ci/ci_comment_density_guard.py
# Comment-encoding guard OK (1149 grandfathered finding(s), no growth)

git diff --check
# clean

Closes #2196.