Fix access violation for MarianTokenizer

[apsonawane]

This pull request updates the CI/CD pipeline configuration and improves the robustness of the tokenizer codebase. The main theme is upgrading macOS build agents from version 13 to 15 across all pipeline YAML files, ensuring compatibility with newer macOS features and Xcode versions. Additionally, there are targeted improvements to tokenization logic, including better state management and input validation, and a minor test expectation update for JPEG decoding.

Pipeline and Build Environment Updates

• Updated all pipeline YAML files (.pipelines/*.yml, tools/ci_build/github/azure-pipeline/*.yml) to use macOS-15 build agents instead of macOS-13, ensuring builds run on the latest macOS infrastructure. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
• Updated default Xcode version in use-xcode-version.yml template from 14.3 to 16.1 to support the latest toolchain and macOS features.

Tokenizer Robustness Improvements

• Added a Reset method to CaseEncoder and ensured it is called before each normalization, preventing state leakage between tokenization calls. [1] [2]
• Added bounds checking and iterator validation to CaseEncoder::Normalize to prevent out-of-bounds errors during normalization.
• Added UTF-8 input validation in SpmUgmTokenizer::ComputeNoOp, returning an error if invalid input is detected.

Testing and CMake Fixes

• Updated JPEG decoder test expectations for Apple platforms to reflect correct output values.
• Fixed a CMake configuration issue in ext_tests.cmake to avoid duplicate PlugIns directory by clearing LIBRARY_OUTPUT_DIRECTORY property.