feat(diagnostic): add support for many namespaces filtering in GetOpts (#28045)

runtime/doc/diagnostic.txt

@@ -371,7 +371,8 @@ Lua module: vim.diagnostic                                    *diagnostic-api*
     A table with the following keys:
 
     Fields: ~
-      • {namespace}?  (`integer`) Limit diagnostics to the given namespace.
+      • {namespace}?  (`integer[]|integer`) Limit diagnostics to one or more
+                      namespaces.
       • {lnum}?       (`integer`) Limit diagnostics to the given line number.
       • {severity}?   (`vim.diagnostic.SeverityFilter`) See
                       |diagnostic-severity|.

runtime/doc/news.txt

@@ -408,6 +408,9 @@ The following changes to existing APIs or features add new behavior.
   "virtual_text" table, which gives users more control over how diagnostic
   virtual text is displayed.
 
+• |vim.diagnostic.get()| and |vim.diagnostic.count()| accept multiple
+  namespaces rather than just a single namespace.
+
 • Extmarks now fully support multi-line ranges, and a single extmark can be
   used to highlight a range of arbitrary length. The |nvim_buf_set_extmark()|
   API function already allowed you to define such ranges, but highlight regions

runtime/lua/vim/diagnostic.lua

@@ -682,6 +682,13 @@ local function get_diagnostics(bufnr, opts, clamp)
   opts = opts or {}
 
   local namespace = opts.namespace
+
+  if type(namespace) == 'number' then
+    namespace = { namespace }
+  end
+
+  ---@cast namespace integer[]
+
   local diagnostics = {}
 
   -- Memoized results of buf_line_count per bufnr
@@ -742,11 +749,15 @@ local function get_diagnostics(bufnr, opts, clamp)
     end
   elseif bufnr == nil then
     for b, t in pairs(diagnostic_cache) do
-      add_all_diags(b, t[namespace] or {})
+      for _, iter_namespace in ipairs(namespace) do
+        add_all_diags(b, t[iter_namespace] or {})
+      end
     end
   else
     bufnr = get_bufnr(bufnr)
-    add_all_diags(bufnr, diagnostic_cache[bufnr][namespace] or {})
+    for _, iter_namespace in ipairs(namespace) do
+      add_all_diags(bufnr, diagnostic_cache[bufnr][iter_namespace] or {})
+    end
   end
 
   if opts.severity then
@@ -785,7 +796,7 @@ end
 --- @param search_forward boolean
 --- @param bufnr integer
 --- @param opts vim.diagnostic.GotoOpts
---- @param namespace integer
+--- @param namespace integer[]|integer
 --- @return vim.Diagnostic?
 local function next_diagnostic(position, search_forward, bufnr, opts, namespace)
   position[1] = position[1] - 1
@@ -1115,8 +1126,8 @@ end
 --- A table with the following keys:
 --- @class vim.diagnostic.GetOpts
 ---
---- Limit diagnostics to the given namespace.
+--- Limit diagnostics to one or more namespaces.
---- @field namespace? integer
+--- @field namespace? integer[]|integer
 ---
 --- Limit diagnostics to the given line number.
 --- @field lnum? integer