[direct-linearize] fix name stack tests

[mattjj]

Now this passes:

JAX_USE_DIRECT_LINEARIZE=1 python tests/name_stack_test.py

See #8395 for an explanation of how the name stack should work. There were three changes we needed to replicate current behavior:

1. push 'jvp' onto the name stack when we trace the function in direct_linearize;
2. to solve the "doubled naming" problem, set the name stack only to be the appropriate suffix when running the tangent part of the computation in LinearizeTrace.process_primitive;
3. when doing the jaxpr-to-jaxpr linearize_jaxpr, be sure to reset the name stack (which was provided in trace_to_jaxpr_dynamic, but linearize_jaxpr doesn't use that and instead uses lower-level stuff).