test(lsp/diagnostic): clearing diagnostics #37759

Problem:
Diagnostic lifecycle invariants (clearing on empty publish and buffer
deletion) were previously implicit and not directly covered by functional
tests, allowing regressions to go unnoticed.

Solution:
Add functional regression tests asserting that diagnostics are cleared
when an LSP server publishes an empty diagnostic set and when the
associated buffer is deleted. Assertions are scoped to the client
diagnostic namespace and use public diagnostic APIs only.

test/functional/plugin/lsp/diagnostic_spec.lua

@@ -208,6 +208,67 @@ describe('vim.lsp.diagnostic', function()
         end)
       )
     end)
+
+    it('clears diagnostics for the client namespace on empty publish', function()
+      local before_clear, after_clear = exec_lua(function()
+        local ns = vim.lsp.diagnostic.get_namespace(client_id)
+
+        -- Publish diagnostics
+        vim.lsp.diagnostic.on_publish_diagnostics(nil, {
+          uri = fake_uri,
+          diagnostics = {
+            _G.make_error('Diagnostic', 0, 0, 0, 0),
+          },
+        }, { client_id = client_id })
+
+        local before_clear = vim.diagnostic.get(diagnostic_bufnr, { namespace = ns })
+
+        -- Publish empty diagnostics
+        vim.lsp.diagnostic.on_publish_diagnostics(nil, {
+          uri = fake_uri,
+          diagnostics = {},
+        }, { client_id = client_id })
+
+        local after_clear = vim.diagnostic.get(diagnostic_bufnr, { namespace = ns })
+
+        return before_clear, after_clear
+      end)
+
+      eq(1, #before_clear)
+      eq(0, #after_clear)
+    end)
+
+    it('clears diagnostics when buffer is deleted', function()
+      local before_delete, after_delete = exec_lua(function()
+        local ns = vim.lsp.diagnostic.get_namespace(client_id)
+
+        -- Publish diagnostics
+        vim.lsp.diagnostic.on_publish_diagnostics(nil, {
+          uri = fake_uri,
+          diagnostics = {
+            _G.make_error('Diagnostic', 0, 0, 0, 0),
+          },
+        }, { client_id = client_id })
+
+        local before_delete = vim.diagnostic.get(diagnostic_bufnr, { namespace = ns })
+
+        -- Avoid deleting the currently displayed buffer (Windows teardown edge case)
+        local scratch = vim.api.nvim_create_buf(false, true)
+        vim.api.nvim_win_set_buf(0, scratch)
+
+        -- Delete the buffer deterministically
+        vim.api.nvim_buf_delete(diagnostic_bufnr, { force = true })
+
+        -- Query remaining diagnostics via valid access path
+        -- (deleted buffer cannot be queried directly)
+        local after_delete = vim.diagnostic.get(nil, { namespace = ns })
+
+        return before_delete, after_delete
+      end)
+
+      eq(1, #before_delete)
+      eq(0, #after_delete)
+    end)
   end)
 
   describe('vim.lsp.diagnostic.on_diagnostic', function()