[border-agent] try getting coap dtls session from ephemeral key manager (#11270)

If the EphemeralKey Manager is enabled, when no active CoAP DTLS
session is found in the Border Agent's mDtlsTransport, it should
attempt to retrieve one managed by the EphemeralKey Manager.

This issue was discovered when using Thread devices, a Border Router ,
and `ot-commissioner` for commissioning with an ephemeral key. The
process is as follows:

Using `ot-commissioner` commit: `8fffd186b28809257bad73eab2459c97240fe535`
Using `openthread` commit: `36a8f71d0693bfb1407fc48444c4da6e7f2d32d6`

1. Using the ephemeral key, the BR and `ot-commissioner` established a
DTLS connection over the Wi-Fi link. This process worked as expected,
and the Border Agent successfully created `mCoapDtlsSession` in the
EphemeralKey Manager.

2. The `ot-commissioner` executed the command `joiner enableall
meshcop J01NU5` to allow joiners to connect.

3. When a Thread node attempted to join the network using `joiner
start J01NU5`, it failed.

The root cause was that when the `BorderAgent` received a CoAP message
from the Thread device and attempted to forward it to
`ot-commissioner` via `HandleTmf<kUriRelayRx>`, it could not find an
active CoAP DTLS session through
`FindActiveCommissionerSession`. Because the `mCoapDtlsSession` in the
EphemeralKey Manager was not managed by the Border Agent’s
`mDtlsTransport`.

src/core/meshcop/border_agent.cpp

@@ -280,6 +280,14 @@ BorderAgent::CoapDtlsSession *BorderAgent::FindActiveCommissionerSession(void)
         }
     }
 
+#if OPENTHREAD_CONFIG_BORDER_AGENT_EPHEMERAL_KEY_ENABLE
+    if ((mEphemeralKeyManager.mCoapDtlsSession != nullptr) &&
+        mEphemeralKeyManager.mCoapDtlsSession->IsActiveCommissioner())
+    {
+        commissionerSession = mEphemeralKeyManager.mCoapDtlsSession;
+    }
+#endif
+
     return commissionerSession;
 }