Do not raise KeyError copying an undecryptable channels_log entry

parsePacketPayload copies message/msg_hash/sender_timestamp/attempt/txt_type from a
prior channels_log entry matched by pkt_hash. When that prior entry was logged while
the channel could not be decrypted it has none of those keys, so a later duplicate of
the same transmission raises KeyError: 'message' and aborts handle_rx for the whole
packet, dropping its RX_LOG_DATA. It triggers when a channel-table refresh lands
between two copies of a flooded channel message.

Copy with .get() so the fields are simply absent instead of raising. Adds a regression
test that reproduces the crash (fails on main, passes here).

tests/unit/test_rxlog_keyerror_regression.py

@@ -0,0 +1,50 @@
+import pytest
+from Crypto.Cipher import AES
+from Crypto.Hash import HMAC, SHA256
+
+from meshcore.meshcore_parser import MeshcorePacketParser
+
+
+@pytest.mark.asyncio
+async def test_duplicate_of_undecryptable_channel_frame_does_not_keyerror():
+    """A channel frame first heard without the key is logged with no "message".
+
+    When a duplicate of it arrives after the key becomes known (a channel-table refresh
+    landing between two copies of a flooded transmission), parsePacketPayload matches the
+    prior, keyless channels_log entry by pkt_hash and copies its fields. That entry has no
+    "message"/"msg_hash"/etc., so `logged["message"]` raised KeyError and aborted handle_rx
+    for the whole packet — dropping its RX_LOG_DATA. It must not raise.
+    """
+    parser = MeshcorePacketParser()
+    parser.decrypt_channels = True
+
+    channel_secret = b"\x01" * 16
+    parser.channels[0] = {
+        "channel_idx": 0,
+        "channel_name": "test",
+        "channel_hash": "ab",
+        "channel_secret": channel_secret,
+    }
+
+    # A valid GRP_TXT (channel) frame: header (route_type=1, payload_type=5), path_byte=0,
+    # then pkt_payload = chan_hash(1) + cipher_mac(2) + AES-ECB ciphertext(16).
+    plaintext = b"\x00\x00\x00\x00\x15hello\x00\x00\x00\x00\x00\x00"
+    assert len(plaintext) == 16
+    encrypted = AES.new(channel_secret, AES.MODE_ECB).encrypt(plaintext)
+    h = HMAC.new(channel_secret, digestmod=SHA256)
+    h.update(encrypted)
+    cipher_mac = h.digest()[:2]
+    pkt_payload = bytes([0xAB]) + cipher_mac + encrypted
+    payload = bytes([0x15, 0x00]) + pkt_payload
+
+    # Simulate the earlier reception, logged while the channel could not be decrypted:
+    # a channels_log entry for this pkt_hash with NO "message"/"msg_hash"/etc.
+    pkt_hash = int.from_bytes(SHA256.new(pkt_payload).digest()[0:4], "little", signed=False)
+    parser.channels_log.append({"pkt_hash": pkt_hash, "chan_hash": "ab"})
+
+    # The duplicate now decrypts and reaches the "found: copy" branch.
+    log_data = await parser.parsePacketPayload(payload, log_data={})
+
+    assert log_data["payload_type"] == 0x05
+    # Copied (absent) from the keyless entry instead of raising KeyError.
+    assert log_data["message"] is None