Mostly removing commented (and replaced) code blocks lingering from the
ctxc semantics work and new typed-msg-spec `MsgType`s handling AND use
the new `._exceptions.pack_from_raise()` helper to construct
`StreamOverrun` msgs.
Deaterz:
- clean out the drain loop now that it's implemented to handle our
struct msg types including the `dict`-msg bits left in as
fallback-reminders, any notes/todos better summarized at the top of
their blocks, remove any `_final_result_is_set()` related duplicate/legacy
tidbits.
- use a `case Error()` block in drain loop with fallthrough to `_:`
always resulting in an rte raise.
- move "XXX" notes into the doc-string for `._deliver_msg()` as
a "rules" section.
- use `match:` syntax for logging the `result_or_err: MsgType` outcome
from the final `.result()` call inside `open_context_from_portal()`.
- generally speaking use `MsgType` type annotations throughout!
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