Android/fix evalue string serialization

[kirklandsign]

Summary

toByteArray() used toString() (returning "EValue@...") instead of
toStr(), and allocated the buffer using char count instead of byte
count. fromByteArray() threw an exception for strings instead of
deserializing them. Fix both directions using a length-prefixed
format, and also fix the misleading "Unknown Tensor dtype" error
message. Add round-trip tests for ASCII, empty, and Unicode strings.

Test plan

CI

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17609

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❌ 3 New Failures, 53 Pending

As of commit f9bc02c with merge base cf08087 ():

NEW FAILURES - The following jobs have failed:

• pull / test-mediatek-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 67292ddf121b8c72edbe557d48f3bbb9f2006941e710c069ecec82e88bb863d5 /exec failed with exit code 2
• pull / test-openvino-linux / linux-job (gh)
  RuntimeError: Command docker exec -t bd838521bfe4491dd359f142c48974ca8e78fd49a9d6d668ca4c7c7bc3d7ecfb /exec failed with exit code 1
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 4ebd804a3a9216ddd00a1bbb46db9a6a825787f9cd12a11aad13fefbeef208ec /exec failed with exit code 3

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[Copilot]

Pull request overview

This PR fixes critical bugs in EValue string serialization/deserialization for the Android ExecuTorch extension. Previously, toByteArray() used toString() (which returned "EValue@..." object representation) instead of toStr(), and fromByteArray() threw an exception for strings. The fix implements a length-prefixed binary format for strings and corrects the error message.

Changes:

• Fixed string serialization to use toStr() instead of toString() and implements length-prefixed format
• Fixed string deserialization to properly decode strings instead of throwing exceptions
• Improved error message from "Unknown Tensor dtype" to "Unknown EValue type code"
• Added comprehensive round-trip tests for ASCII, empty, Chinese, and emoji strings

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
extension/android/executorch_android/src/main/java/org/pytorch/executorch/EValue.java	Fixes string serialization/deserialization logic and error messages
extension/android/executorch_android/src/test/java/org/pytorch/executorch/EValueTest.kt	Adds comprehensive tests for string serialization round-trips including Unicode

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