Use safe_numel() in et

[lucylq]

Stack from ghstack (oldest at bottom):

• -> Use safe_numel() in et #19075
• Add safe_numel() #19074

The wasm bindings' assert_valid_numel() helper previously computed numel without overflow detection, letting a malicious tensor shape wrap to a small value that bypassed the data-size check. Replace with safe_numel() (returns Result<ssize_t>) and propagate the error via THROW_IF_ERROR as a JavaScript exception, matching the rest of the wasm error plumbing.

Authored with Claude.

Differential Revision: D102082911

[pytorch-bot]

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❌ 1 Cancelled Job, 1 Pending, 4 Unrelated Failures

As of commit 75530a3 with merge base eef7921 ():

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-editable / macos / macos-job (gh)
  ##[error]The operation was canceled.

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-selective-build-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Test Vulkan Backend / test-vulkan / package-golden-artifacts (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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