GH-49973: [C++] Fix Gandiva string length checks

[puneetdixit200]

Rationale

Fixes remaining Gandiva string length safety issues from GH-49973.

What changed

• Uses checked multiplication before addition in quote_utf8 and to_hex_binary.
• Rejects negative valid input lengths in concat_ws before allocation/copy.
• Adds targeted regression tests for overflow and negative length handling.

Verification

• git diff --check
• ctest --test-dir cpp\build-gandiva-vs-conda-zlib -C Release -R gandiva-precompiled-test --output-on-failure

• GitHub Issue: [C++] Fix remaining overflow and negative length handling issues in Gandiva string functions #49973

AI assistance disclosure

AI assistance was used to inspect GH-49973, reason about the overflow and negative-length paths, and draft the change and PR text. The changed code and tests were reviewed for the specific behavior described in the issue.

[Copilot]

Pull request overview

This PR addresses remaining Gandiva string length safety issues (GH-49973) by preventing signed integer overflow during output-size computation and by rejecting negative “valid” input lengths before allocation/copy, with focused regression tests to prevent reintroduction.

Changes:

• Fix potential signed overflow in quote_utf8 and to_hex_binary by using MultiplyWithOverflow before AddWithOverflow for allocation sizing.
• Add explicit negative-length validation and overflow-safe length accumulation for concat_ws_* before allocation/memcpy.
• Add regression tests covering overflow sizing and negative-length handling.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cpp/src/gandiva/precompiled/string_ops.cc	Adds checked arithmetic for allocation sizing and validates negative lengths in concat_ws to prevent unsafe allocation/copy paths.
cpp/src/gandiva/precompiled/string_ops_test.cc	Adds targeted tests asserting correct error behavior for overflow sizing and negative-length inputs.

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