[zxdb_cli][shared] Split protocol unit tests

Extract ProtocolTestCase into shared/tests/helpers.py and split the
command schema tests out of test_protocol.py into 9 domain-scoped test
modules under shared/protocol/tests/ (lifecycle, attach/detach,
event/state, execution, breakpoints, tracepoints, stack/async-backtrace,
process inspection, and evaluate/console).

Keep test_protocol.py focused on generic request dispatch, response
envelope deserialization, and dynamic registration verification across
BaseRequest subclasses.

Test: fx test scripts/debug/zxdb_cli/shared
Change-Id: I9c4e72a81d604b3f5e891a2047bc63928174e501
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1921132
Commit-Queue: Jacob Rutherford <jruthe@google.com>
Fuchsia-Auto-Submit: Jacob Rutherford <jruthe@google.com>
Lint: android-build-ayeaye@system.gserviceaccount.com <android-build-ayeaye@system.gserviceaccount.com>
Reviewed-by: Dorian Lee <dorianlee@google.com>
Risk: Fuchsia CL Risk (Advisory) <groupagent-fuchsia-risk@google.com>