#27539 Cleanup external data path validation

Description

• Updates the ValidateExternalData() function:
  • Resolves symlinks when validating external data path for models loaded from memory.
    • Previously only did a lexical check that did not resolve symlinks. Now, we resolve symlinks.
  • Now requires external data path to exist.
  • Replace the (base_dir, model_dir) function parameters with just model_dir. base_dir was always derived from model_dir.
  • Skip validation for WASM builds that do not have a filesystem.
  • Return Status instead of throwing exceptions when std::filesystem functions fail.
• Updates Graph::ConvertInitializersIntoOrtValues():
  • Prevents unnecessary calls to ValidateExternalData() for external data paths that have already been validated.

Motivation and Context