assistant2: Add thread persistence (#23582)

This PR adds persistence for threads in Assistant2.

Threads are now persisted to an LMDB database.

Release Notes:

- N/A

Cargo.lock

@@ -456,6 +456,7 @@ dependencies = [
  "futures 0.3.31",
  "fuzzy",
  "gpui",
+ "heed",
  "html_to_markdown",
  "http_client",
  "indoc",
@@ -470,6 +471,7 @@ dependencies = [
  "menu",
  "multi_buffer",
  "parking_lot",
+ "paths",
  "picker",
  "project",
  "prompt_library",
@@ -490,7 +492,6 @@ dependencies = [
  "time",
  "time_format",
  "ui",
- "unindent",
  "util",
  "uuid",
  "workspace",