Make thread context wait on detailed summary + remove "Summarizing context..." (#29564)

This moves summarization task management out of `context_store`. The
code there was draining a Vec of tasks to block on, but this is no
longer a good fit for message_editor's context loading. It needs to be
able to repeatedly await on the thread summarization tasks involved in
the context.

Discussed with Danilo, and he thinks it'd be good to remove the current
"Summarizing context" anyway since it causes layout shift. If message
send is blocked on summarizing, the pulsing context pill is sufficient
for now. This UI change made this overall change more straightforward.

Release Notes:

- N/A

Cargo.lock

@@ -94,6 +94,7 @@ dependencies = [
  "parking_lot",
  "paths",
  "picker",
+ "postage",
  "project",
  "prompt_store",
  "proto",