HTTP API: 404 for a chat id the list of chats leaves out (closes #5)

The chat client keeps contact records that GET /api/v1/chats does not
list, such as the bot's own profile (1 on a new profile) and a contact
it creates itself (2), and reads or sends in them when asked. The
message endpoints now look the id up, through chatID in
internal/api/chats.go, in the same list the chats endpoint answers
with, and answer 404 for any id not in it. The webhook endpoints can
call chatID too.

Model: opus-5-5

internal/api/messages.go

@@ -9,7 +9,6 @@ import (
 	"strconv"
 	"time"
 
-	"github.com/go-chi/chi/v5"
 	"sneak.berlin/go/simplexcalc/internal/simplex"
 )
 
@@ -18,9 +17,6 @@ const (
 	// request for its messages reads.
 	defaultCount = 20
 	maxCount     = 100
-
-	// noSuchChat answers a request naming a chat the bot does not have.
-	noSuchChat = "no such chat"
 )
 
 // message is how the API shows a message, wherever it shows one.
@@ -69,10 +65,8 @@ func (h *handlers) handleMessages() http.HandlerFunc {
 	}
 
 	return func(w http.ResponseWriter, r *http.Request) {
-		contactID, ok := chatID(r)
+		contactID, ok := h.chatID(w, r)
 		if !ok {
-			h.respondError(w, http.StatusNotFound, noSuchChat)
-
 			return
 		}
 
@@ -116,10 +110,8 @@ func (h *handlers) handleSend() http.HandlerFunc {
 	}
 
 	return func(w http.ResponseWriter, r *http.Request) {
-		contactID, ok := chatID(r)
+		contactID, ok := h.chatID(w, r)
 		if !ok {
-			h.respondError(w, http.StatusNotFound, noSuchChat)
-
 			return
 		}
 
@@ -170,9 +162,9 @@ func (h *handlers) handleSend() http.HandlerFunc {
 }
 
 // respondChatError answers a request the chat client did not serve: 404
-// for a chat the bot does not have, 409 for a contact who cannot receive
-// messages, 413 for a text too long to send, and 500 with sentence for
-// anything else, which is logged as what.
+// for a contact removed after chatID found it, 409 for one who cannot
+// receive messages, 413 for a text too long to send, and 500 with
+// sentence for anything else, which is logged as what.
 func (h *handlers) respondChatError(
 	w http.ResponseWriter, err error, what, sentence string,
 ) {
@@ -189,14 +181,6 @@ func (h *handlers) respondChatError(
 	}
 }
 
-// chatID returns the chat id in the request's path, and false if it is
-// not one: a chat's id is its contact's, a positive whole number.
-func chatID(r *http.Request) (int64, bool) {
-	id, err := strconv.ParseInt(chi.URLParam(r, "id"), 10, 64)
-
-	return id, err == nil && id > 0
-}
-
 // messageCount returns the request's count, defaultCount if it has none,
 // and false if it is anything but a whole number from 1 to maxCount. The
 // query is parsed here because r.URL.Query drops a pair it cannot