Fixes for Dandi file access

[samuelbray32]

• Fixes Error accessing dandi-loaded raw files #1473

  • Ensure raw nwb files access through dandi are stored in __open_nwb_files using the copy filename

• Fixes Error for fetching non-local files #1472

  • Avoid error-triggering axess of externals table when fetching nwb files

• Fixes Error in fetch_nwb due to inconsistent path type #1428

  • Ensures all cases of AnalysisNwbfile.get_abs_path return a string object

Checklist:

• If this PR should be accompanied by a release, I have updated the CITATION.cff
• If this PR edits table definitions, I have included an alter snippet for release notes.
• If this PR makes changes to position, I ran the relevant tests locally.
• If this PR makes user-facing changes, I have added/edited docs/notebooks to reflect the changes
• I have updated the CHANGELOG.md with PR number and description.

[Copilot]

Pull Request Overview

This pull request fixes three issues related to NWB file access through Dandi:

• Fix #1473: Refactored _open_nwb_file to handle raw NWB file path resolution internally, ensuring files are cached using the correct key
• Fix #1472: Removed redundant code that forced nwb2load_filepath into fetch attributes, avoiding unnecessary external table access
• Fix #1428: Added str() conversion to ensure AnalysisNwbfile.get_abs_path consistently returns string objects when fetching from schema

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/spyglass/utils/nwb_helper_fn.py	Centralized Dandi path resolution logic in _open_nwb_file to properly handle both direct file paths and raw paths, and simplified get_nwb_file by consolidating path checking conditions
src/spyglass/utils/mixins/analysis.py	Added explicit string conversion for path returned from schema externals table to ensure consistent string return type
src/spyglass/utils/dj_helper_fn.py	Removed redundant code that unnecessarily added nwb2load_filepath to fetch attributes, as this attribute is populated in the loop regardless

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