feat: define linked Agent Session message page contract

[haesookimDev]

Summary

• Add strict SDK models for Workflow's journal-linked completed Agent Session message page and 409 response.
• Reconcile sparse message sequences with a session-bound turn cursor, duplicate-page handling, and fail-closed cursor checks.
• Document that these pages do not represent complete conversation history or advance the event replay cursor.

Verification

• PYTHONPATH=src .venv/bin/python -m pytest -q --ignore=tests/test_oracle_ssl_dsn.py --ignore=tests/test_tibero_dsn.py: 776 passed, 1 skipped.
• Full test collection is blocked by the existing missing optional sqlalchemy dependency in those two unrelated DSN tests.
• uv build --offline --wheel passed; wheel inspection confirmed the new model and exports.
• git diff --check passed.

Integration

Target feat/cross-platform-session. Parent PR #53 remains Draft. This SDK contract is tested from source and the local wheel; no SDK release is performed.