feat(papers): add pubmed_search, fetch_pubmed, fetch_doi to hf_papers tool

[lucianfialho]

Summary

Extends hf_papers with three new operations covering biomedical and DOI-based literature that the current tool can't reach:

• pubmed_search — keyword search over PubMed via NCBI E-utilities (esearch + esummary). Covers clinical, biomedical, and pharmacological literature not indexed on arXiv/HF Hub.
• fetch_pubmed — fetch abstract + metadata for a PMID via efetch XML. Returns title, authors, journal, year, DOI link.
• fetch_doi — fetch metadata + abstract for any DOI via Crossref API. Covers bioRxiv, medRxiv, PsyArXiv, and journal articles. Useful when you have a DOI from a citation graph result but no arXiv ID.

Why this gap matters

The existing search operation is HF/arXiv-tuned — great for CS/ML. But research_tool increasingly needs to pull biomedical papers (clinical benchmarks, neuroscience, drug interaction studies). These live on PubMed/DOI space, not arXiv. This PR makes hf_papers the single tool for any literature source.

Implementation

• No new dependencies — httpx (already present) + stdlib xml.etree.ElementTree
• Follows the existing _op_* / ToolResult pattern exactly
• Registered in both _OPERATIONS and HF_PAPERS_TOOL_SPEC (enum + parameter docs)
• pmid and doi parameters added to schema with removeprefix normalization (accepts pmid:38903003 or 38903003)

Example usage

# Search biomedical literature
hf_papers(operation="pubmed_search", query="psilocybin anxiety clinical trial", limit=5)

# Fetch by PMID
hf_papers(operation="fetch_pubmed", pmid="38903003")

# Fetch a bioRxiv preprint by DOI
hf_papers(operation="fetch_doi", doi="10.1101/2023.12.15.571821")

Closes #93

Test plan

• pubmed_search returns results for a known biomedical query
• fetch_pubmed returns abstract for pmid:38903003 (known good)
• fetch_doi returns metadata for a bioRxiv DOI
• fetch_doi handles a paywalled journal DOI gracefully (returns Crossref metadata even if full text unavailable)
• Existing operations unaffected (schema enum extended, not replaced)