Ignore errors when releasing RPC sessions.

I'm not sure why / when these happen yet, as I can't reproduce quite
yet, but doing this is "safe" in the sense that on the Lean side all
that will happen is some bit of leaking memory.

Let's choose that over an error for now until we figure out what else we
should be doing (or aren't doing).

lua/lean/rpc.lua

@@ -23,7 +23,7 @@ local rpc = {}
 ---@field client vim.lsp.Client
 ---@field uri string
 ---@field connected? boolean
----@field session_id? string
+---@field session_id? integer
 ---@field connect_err? string
 ---@field on_connected Condvar
 ---@field keepalive_timer? uv_timer_t
@@ -92,17 +92,39 @@ function Session:close()
   self:close_without_releasing()
 end
 
+---A notification to release remote references. Should be sent by the client when it no longer needs
+---`RpcRef`s it has previously received from the server. Not doing so is safe but will leak memory.
+---@class RpcReleaseParams
+---@field uri lsp.DocumentUri
+---@field sessionId integer
+---@field refs RpcRef[]
+
 ---@param refs RpcRef[]
 function Session:release_now(refs)
   vim.list_extend(self.to_release, refs)
   if #self.to_release == 0 or self:is_closed() then
     return
   end
-  self.client.notify('$/lean/rpc/release', {
+
+  log:debug {
+    message = 'releasing RPC refs',
+    uri = self.uri,
+    refs = refs,
+  }
+
+  ---@type RpcReleaseParams
+  local params = {
     uri = self.uri,
     sessionId = self.session_id,
     refs = self.to_release,
-  })
+  }
+  local succeeded = pcall(self.client.notify, '$/lean/rpc/release', params)
+  if not succeeded then
+    log:warning {
+      message = 'unable to release RPC session, which leaks a bit of memory',
+      params = params,
+    }
+  end
   self.to_release = {}
 end