Fix thread safety of LLM token buffer in Android JNI

[kirklandsign]

Summary

The UTF-8 token buffer was a global variable shared across all ExecuTorchLlmJni instances, causing token corruption when multiple LlmModule instances run generate() concurrently. Move the buffer to a local variable in generate() captured by the callback lambda, giving each call its own isolated buffer. This matches the pattern already used by the ASR JNI layer.

Test plan

CI

[pytorch-bot]

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❌ 4 New Failures

As of commit c02b379 with merge base cf08087 ():

NEW FAILURES - The following jobs have failed:

• pull / test-mediatek-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t b35eb99d214204bb8bc5bb42dfac958f1f70b57efdc93c31f648366eecb0280e /exec failed with exit code 2
• pull / test-openvino-linux / linux-job (gh)
  RuntimeError: Command docker exec -t e147695d66beb62a003ea6c01c441cc5a12bdf65663f6a76ccae673490f795a6 /exec failed with exit code 1
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 111221f396618299c0aeef2057e4612f6bbb37c3195e5f7ae4916e22e6f3df5a /exec failed with exit code 3
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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