fix(tests/e2e-backends): bump ctx_size for llama-cpp transcription

Qwen3-ASR-0.6B encodes the jfk.wav fixture into 777 audio tokens via
its mmproj, but the test harness defaulted BACKEND_TEST_CTX_SIZE to
512, so llama.cpp server rejected every transcription request with
"request (777 tokens) exceeds the available context size (512 tokens)".

Set BACKEND_TEST_CTX_SIZE=2048 on the llama-cpp transcription target
only — sherpa-onnx and vibevoice transcription targets don't go
through llama.cpp's slot/n_ctx and weren't failing.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude:claude-opus-4-7 [Claude Code]