search_uniprot_entity

    QUERY SYNTAX:
    - Simple keyword: "rubisco"
    - Field-specific: "field:value"  (e.g., "gene:BRCA1", "protein_name:rubisco")
    - Boolean operators: AND, OR, NOT  (e.g., "gene:TP53 AND organism_id:9606")
    - Grouping with parentheses: "((gene:CTNNB1) AND (taxonomy_id:9606))"
    - Wildcards (* suffix): "gene:PRO*" matches any gene starting with PRO
    - Ranges: "length:[1000 TO 2000]" or open-ended "length:[5000 TO *]"

    KEY QUERY FIELDS:
    Identity / Name:
      accession          UniProt primary accession (e.g., "accession:P04637")
      id                 UniProt entry name / mnemonic (e.g., "id:P53_HUMAN")
      protein_name       Protein name, including synonyms (e.g., "protein_name:rubisco")
      gene               Gene name with wildcard support (e.g., "gene:BRCA*")
      gene_exact         Exact gene name match (e.g., "gene_exact:TP53")
      ec                 Enzyme Commission number (e.g., "ec:1.1.1.1")

    Taxonomy:
      organism_id        NCBI taxonomy ID (e.g., "organism_id:9606" for human,
                         "organism_id:10090" for mouse)
      organism_name      Organism scientific or common name
      taxonomy_id        Taxon ID including all descendants
      lineage            Taxonomic lineage keyword

    Annotation status:
      reviewed           true = Swiss-Prot (manually reviewed),
                         false = TrEMBL (automatically annotated)
                         ALWAYS add "reviewed:true" when seeking high-quality entries.

    Sequence properties:
      length             Sequence length as a range (e.g., "length:[100 TO 500]")
      mass               Molecular mass in Daltons (range supported)
      existence          Protein existence level: 1 (protein), 2 (transcript),
                         3 (homology), 4 (predicted), 5 (uncertain)

    Functional annotation:
      keyword            UniProt keyword name (e.g., "keyword:Kinase")
      keyword_id         UniProt keyword ID (e.g., "keyword_id:KW-0418")
      function           Function free-text annotation
      family             Protein family (e.g., "family:globin")
      organelle          Subcellular organelle (e.g., "organelle:chloroplast")
      cc_subcellular_location  Subcellular location comment

    Cross-references:
      database           Database cross-reference (e.g., "database:PDB")
      xref               Cross-reference ID (e.g., "xref:pdb-1A2B")
      chebi              ChEBI ID (e.g., "chebi:15422")
      interactor         UniProt accession of interacting protein

    Literature:
      lit_author         Author surname (e.g., "lit_author:Smith")
      lit_pubmed         PubMed ID
      lit_doi            DOI

    EXAMPLES (structured queries):
      # Reviewed human TP53 protein
      "gene_exact:TP53 AND organism_id:9606 AND reviewed:true"

      # All human kinases manually reviewed
      "keyword:Kinase AND organism_id:9606 AND reviewed:true"

      # EGFR in human or mouse
      "gene_exact:EGFR AND (organism_id:9606 OR organism_id:10090) AND reviewed:true"

      # Long chloroplast proteins (>= 5000 aa) in any organism
      "organelle:chloroplast AND length:[5000 TO *]"

      # Proteins with PDB structures involved in apoptosis
      "database:PDB AND keyword:Apoptosis AND organism_id:9606 AND reviewed:true"

      # Proteins encoded by gene names starting with "PIK3"
      "gene:PIK3* AND organism_id:9606 AND reviewed:true"

limit (int): The maximum number of results to return. Default is 20.