fix(coderd): sanitize Anthropic provider tool history (#24706)

Anthropic can reject replayed chat histories when a provider-executed
tool call, such as `web_search`, is present without its matching
provider result block.

This sanitizes unpaired Anthropic provider-executed tool calls during
prompt reconstruction, before Anthropic requests, and before persistence
so existing poisoned histories can continue and new malformed turns are
not stored.

Resolves: CODAGT-259

> Mux is acting on Mike's behalf.

coderd/exp_chats_test.go

@@ -1066,7 +1066,7 @@ func TestListChats(t *testing.T) {
 					}
 				}
 				return false
-			}, testutil.WaitShort, testutil.IntervalFast)
+			}, testutil.WaitLong, testutil.IntervalFast)
 		}
 
 		// Fetch first page with limit=2.