refactor(lsp): convert diagnostics to capability framework #40433

Problem:
Diagnostic tracking used a separate bufstates table and manual
LspDetach/LspNotify autocmd management via _enable()/_refresh(),
duplicating the lifecycle logic already provided by the Capability
framework. This caused inconsistencies in how client attach/detach and
buffer teardown were handled compared to other LSP features.

Solution:
Replace the ad-hoc bufstate tracking and _enable/_refresh pattern in
vim.lsp.diagnostic with a proper Diagnostics subclass of Capability.
This cleans up a few random places in the main lsp module and client
module that were poking the diagnostics. It also fixes some pre-existing
bugs and inconsistencies that were discovered:

- Refresh diagnostics immediately on attach instead of lazily by the
  first didOpen/didChange notification
- Fix Capability.active lookup in M.enable() to key by it_bufnr instead
  of the filter bufnr
- Set lsp defaults before calling the _text_document_did_open_handler in
  Client:on_attach() so defaults are there before any lsp notification
  occurs
- Log (and return early) on any error from a diagnostic request result
  instead of only returning early for server cancelled errors

runtime/lua/vim/lsp/client.lua

@@ -554,11 +554,12 @@ function Client:initialize()
   end
 
   -- HACK: Capability modules must be loaded
-  require('vim.lsp.semantic_tokens')
   require('vim.lsp._folding_range')
-  require('vim.lsp.inline_completion')
+  require('vim.lsp.diagnostic')
   require('vim.lsp.document_color')
+  require('vim.lsp.inline_completion')
   require('vim.lsp.linked_editing_range')
+  require('vim.lsp.semantic_tokens')
 
   local init_params = {
     -- The process Id of the parent process that started the server. Is null if
@@ -1187,10 +1188,10 @@ end
 --- Useful for buffer-local setup.
 --- @param bufnr integer Buffer number
 function Client:on_attach(bufnr)
-  self:_text_document_did_open_handler(bufnr)
-
   lsp._set_defaults(self, bufnr)
 
+  self:_text_document_did_open_handler(bufnr)
+
   api.nvim_exec_autocmds('LspAttach', {
     buf = bufnr,
     modeline = false,
@@ -1439,9 +1440,6 @@ function Client:_on_detach(bufnr)
   self:_text_document_did_close_handler(bufnr)
 
   self.attached_buffers[bufnr] = nil
-
-  local namespace = lsp.diagnostic.get_namespace(self.id)
-  vim.diagnostic.reset(namespace, bufnr)
 end
 
 --- Reset defaults set by `set_defaults`.