Simplifies the data structures involved in agent context by removing
caching and limiting the use of ContextId:
* `AssistantContext` enum is now like an ID / handle to context that
does not need to be updated. `ContextId` still exists but is only used
for generating unique `ElementId`.
* `ContextStore` has a `IndexMap<ContextSetEntry>`. Only need to keep a
`HashSet<ThreadId>` consistent with it. `ContextSetEntry` is a newtype
wrapper around `AssistantContext` which implements eq / hash on a subset
of fields.
* Thread `Message` directly stores its context.
Fixes the following bugs:
* If a context entry is removed from the strip and added again, it was
reincluded in the next message.
* Clicking file context in the thread that has been removed from the
context strip didn't jump to the file.
* Refresh of directory context didn't reflect added / removed files.
* Deleted directories would remain in the message editor context strip.
* Token counting requests didn't include image context.
* File, directory, and symbol context deduplication relied on
`ProjectPath` for identity, and so didn't handle renames.
* Symbol context line numbers didn't update when shifted
Known bugs (not fixed):
* Deleting a directory causes it to disappear from messages in threads.
Fixing this in a nice way is tricky. One easy fix is to store the
original path and show that on deletion. It's weird that deletion would
cause the name to "revert", though. Another possibility would be to
snapshot context metadata on add (ala `AddedContext`), and keep that
around despite deletion.
Release Notes:
- N/A
