UDS SecurityAccess/SessionControl timing out in Python (udsoncan/isotp) when ECU simulation is in CANoe (CAPL)

There is a CAPL function called CanTpSetPadding. This function can be used to configure CANoe's padding behavior for a ISO-TP connection:

In your case:

CanTpSetPadding(gHandle, -1)

-1 here means no padding If you, e.g. use 0xAA instead of -1, the the frames will be padded with 0xAA

You have used 0x00 which means pad with 0x00 and is exactly what you have observed.

• Why does CanTpSendData(gHandle, resp, 6) still result in an 8-byte CAN frame?

Underlying Diagnostic over CAN (DoCAN) protocol that handles CAN packets segmentation for UDS by default expects CAN frames with DLC 8 or longer. There are two mechanisms to handle that, both are described here: https://uds.readthedocs.io/en/stable/pages/knowledgebase/can.html#data-field

In short words, here Data Padding is used and every CAN frame has DLC equal 8 (or more). If some of the bytes are unused, then filler byte value (in your case 0x00) is added at the end of the frame to fill it to full 8 bytes.

• How can I make CANoe’s CAPL simulation send exactly the number of UDS bytes I specify (without padding), so udsoncan/isotp accepts it?

You can reconfigure udsoncan/isotp to accept these frames.
I'm pretty sure that if you set RXPADDING flag (https://can-isotp.readthedocs.io/en/latest/isotp/socket.html#isotp.socket.flags.RX_PADDING) then udsoncan should be able to receive your messages.