fix: convert published_date str to datetime in zenodo and hal connectors

[dli1986]

Bug: search_zenodo and search_hal raise 'str' object has no attribute 'isoformat' when results are returned via search_papers(sources="all").

Root cause: Both connectors assign a plain string to Paper.published_date, but Paper.to_dict() calls .isoformat() on it, which only works on datetime objects.

Fix: Import datetime in both files and parse the date string into a datetime object before constructing Paper(). Falls back to year-only or None if parsing fails.
error message:
"errors": {
"zenodo": "'str' object has no attribute 'isoformat'",
"hal": "'str' object has no attribute 'isoformat'"
},

[Copilot]

Pull request overview

Fixes a serialization bug where Paper.published_date was set to a string in the Zenodo and HAL connectors, causing .isoformat() failures when results are serialized (e.g., via search_papers(sources="all")).

Changes:

• Parse Zenodo publication_date strings into datetime (with year-only fallback).
• Parse HAL publication/submission dates into datetime, and pass datetime (not str) into Paper(published_date=...).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
paper_search_mcp/academic_platforms/zenodo.py	Adds datetime parsing for Zenodo publication dates before building Paper.
paper_search_mcp/academic_platforms/hal.py	Adds datetime parsing for HAL dates and changes published_date to pass a datetime into Paper.

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