fix: stop disconnecting from coderd early and record disconnect correctly #21250
+180
−128
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.

fixes coder/internal#1196
The above issue exposes two different bugs in Coder.
In the agent, there is a race where if the agent is closed while starting up networking, it will erroneously disconnect from Coderd, which delays or breaks writing final status and logs.
In Coderd, there is a bug where we don't properly record the latest agent disconnection time if the agent had previously disconnected. This causes us to report the agent status as "Connected" even after it has disconnected up until the inactivity timeout fires.
This PR fixes both issues.
It also slightly reworks when we send workspace updates based on connection and disconnection. Previously we would send two updates when the agent connected in certain circumstances, even though the status would be the same in both (only times changed). Now we universally only send one on connect, and then another on disconnect.