feat(oauth): add OAuthClient:clear_tokens() method

Problem: no way to wipe just the token file while keeping credentials
intact — `_wipe()` removed both.

Solution: add `clear_tokens()` that removes only the token file.

lua/pending/sync/oauth.lua

@@ -496,6 +496,11 @@ function OAuthClient:_wipe()
   os.remove(vim.fn.stdpath('data') .. '/pending/google_credentials.json')
 end
 
+---@return nil
+function OAuthClient:clear_tokens()
+  os.remove(self:token_path())
+end
+
 ---@param opts { name: string, scope: string, port: integer, config_key: string }
 ---@return pending.OAuthClient
 function M.new(opts)