whisper : mask padded K/V in flash-attention encoder and cross-attention

[gooofy]

With flash attention enabled, the encoder self-attention and the decoder cross-attention view their K/V buffers padded to GGML_PAD(n_ctx, 256) but only write n_ctx rows, and call ggml_flash_attn_ext with a null mask. The unwritten pad rows are attended at full weight; their content is whatever the buffer holds - zeros on a fresh state, or leftovers of previous whisper_full calls with a different audio_ctx (layer strides il*n_ctx_pad shift between window sizes, so runs overwrite each other's pad regions).

On a long-running server this makes transcriptions depend on request history: e.g. large-v3-turbo with audio_ctx=250 on a 3 s clip produced 5x sentence duplication on a fresh state, 2x after one audio_ctx=500 request, and clean output after full-window requests. The default full window is affected too (36 unmasked rows out of 1536), just diluted enough to go unnoticed.

Fix: give both call sites a proper mask input (mirroring the decoder self-attention KQ_mask) with -INFINITY over [n_ctx, n_ctx_pad), filled in whisper_encode_internal / whisper_decode_internal. Outputs at reduced audio_ctx are now deterministic regardless of prior requests; timing is unchanged.