Add bounds validation for FreeCall value_index in Method

[lucylq]

The FreeCall instruction handler directly indexes into values_ without bounds checking, enabling OOB memory access via malicious PTE files. This contrasts with JumpFalseCall which validates its index at init time, and MoveCall which uses bounds-checked accessors at execution time.

Add init-time validation for FreeCall matching the JumpFalseCall pattern, and switch execution-time access to use the bounds-checked mutable_value() accessor for defense in depth.

This PR was authored with the assistance of Claude.

[pytorch-bot]

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❌ 2 New Failures, 1 Unrelated Failure

As of commit f9e7d86 with merge base 6c02866 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner (gh)
  >>> Lint for runtime/executor/method.cpp:
• pull / test-binary-size-linux-gcc / linux-job (gh)
  /pytorch/executorch/kernels/portable/cpu/op_convolution.cpp:175:41: error: comparison of integer expressions of different signedness: ‘ssize_t’ {aka ‘long int’} and ‘size_t’ {aka ‘long unsigned int’} [-Werror=sign-compare]

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• Test CUDA Windows Export and E2E / test-model-cuda-windows-e2e (nvidia, diar_streaming_sortformer_4spk-v2, non-quantized) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)

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