fix: diagnose naive identifiers on graph templates at create

[dex0shubham]

Problem

validate_identifiers_fields accepts either identifier schema (NaiveIdentifierSchema | GraphIdentifiersSchema), but after the autotype gate it reads identifiers.entity_id unguarded. NaiveIdentifierSchema only carries item_id, so a graph-family template that uses the naive block dies with a bare attribute error:

# a type: graph template whose identifiers block is `item_id: name`
Template.create("my_graph.yaml", "en", llm_client=llm, embedder=emb)
# AttributeError: 'NaiveIdentifierSchema' object has no attribute 'entity_id'

That is the same unhelpful-crash shape the 0.10.5 load-time check was added to replace — the author of the template gets no hint about what to change.

The right diagnosis already exists in the template validator (_check_identifiers, HE-T003: " templates require entity_id, relation_id, and relation_members (not item_id)"), but he template validate is opt-in while Template.create is on every path.

Fix

Guard the graph field checks with an isinstance test and raise the HE-T003 wording as a ValueError. One check in the shared validator covers all five graph-family creators.

ValueError: graph templates require identifiers.entity_id, relation_id, and relation_members (not item_id)

Verified every template in the Gallery still constructs (0 failures).

Tests

tests/template_engine/test_parser_identifiers.py::TestGraphTemplateWithNaiveIdentifiers:

• test_naive_identifiers_rejected_with_guidance — naive identifiers on a graph type raise the guidance message (fails pre-fix with AttributeError).
• test_graph_identifiers_still_accepted — a well-formed graph block still passes, so the guard cannot over-reject.

ruff check / ruff format --check clean; tests/template_engine/ 105 passed, full suite 724 passed (the 3 TestGoogleGeminiProvider failures are pre-existing on main without the optional google extra).