Feature/end to end test tunneled mode

[dsilhavy]

Adds a test for transmission / reception when tunneled mode is configured as discussed in #55 (review). Detailed workflow:

• Implements a reusable function run_end_to_end_scenario that receives a configuration object to decide if tunneled mode should be applied or not
• For tunneling: The transmitter itself tunnels by wrapping each FLUTE payload in a synthetic inner IPv4+UDP packet and sending it to the tunnel endpoint, and the test’s bridge thread de-tunnels by receiving that packet, validating the inner headers, stripping them off, and forwarding only the original FLUTE bytes to the receiver.
• The assertions verify that transmission completed, a file was received, the received metadata and payload exactly match the expected values, and in tunneled mode that the bridge actually processed packets without errors and kept the encapsulated payload within the configured size limit.

Note: The main code is AI generated with a few manual adjustements