[tmf] validate method as POST centrally in resource handlers (#12661)

This commit updates the central CoAP resource handlers in TMF agents
(`Agent::HandleResource`, `BackboneTmfAgent::HandleResource`, and
`Manager::CoapDtlsSession::HandleResource`) to verify that the
incoming request method is a POST request. If the URI is recognized
but the method is not POST, a `kCodeMethodNotAllowed` response is
now sent.

Since all TMF requests are now guaranteed to be POST requests before
reaching their specific handlers, the `IsPostRequest()` checks in
individual handlers are removed. Additionally, the
`IsConfirmablePostRequest()` and `IsNonConfirmablePostRequest()`
helper methods in `Coap::Message` are removed and their usages are
simplified to `IsConfirmable()` and `IsNonConfirmable()` in the
respective handlers.

src/core/thread/tmf.cpp

@@ -72,6 +72,12 @@ bool Agent::HandleResource(const char *aUriPath, Msg &aMsg)
     bool didHandle = true;
     Uri  uri       = UriFromPath(aUriPath);
 
+    if ((uri != kUriUnknown) && !aMsg.IsPostRequest())
+    {
+        IgnoreError(SendAckResponse(aMsg, ot::Coap::kCodeMethodNotAllowed));
+        ExitNow();
+    }
+
 #define Case(kUri, Type)                   \
     case kUri:                             \
         Get<Type>().HandleTmf<kUri>(aMsg); \
@@ -159,6 +165,7 @@ bool Agent::HandleResource(const char *aUriPath, Msg &aMsg)
 
 #undef Case
 
+exit:
     return didHandle;
 }