TN-57663 - Render retained Graph PDFs by document id

[KirillTonic]

Summary

• add SubjectGraph.render_pdf(document_id) over the retained-source Graph PDF render endpoint
• keep the random-seed request header contract covered while returning the rendered PDF bytes directly
• document the precise retained-PDF contract: V5 styles are stored only for initial PII mentions and mentions added by later Graph enrichment, not for every recognized word
• support the coordinated Solar and Protege pilot implementation listed below

Test plan

• PYTHONPATH=. pytest -q tests/tests/subject_graph_tests/test_render_pdf.py
• GitHub ruff check passes on the latest branch head
• full-corpus retained-source PDF rendering is being exercised by the Protege pilot end-to-end run

Jira: https://tonic-ai.atlassian.net/browse/TN-57663

Related branches and PRs

• Solar: tn-57663-preserve-pdf-layout-in-graph-synthesis — https://github.com/TonicAI/solar/pull/3424
• Textual Python SDK: tn-57663-render-retained-graph-pdfs — TN-57663 - Render retained Graph PDFs by document id #123
• Protege pilot: tn-57663-use-retained-pdf-sources — https://github.com/TonicAI/protege-pilot/pull/1