Cleanup external data path validation (#27539)
### 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
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
