fix: enforce ownership on conversation write endpoints

[yaodong-shen]

Summary

• propagate the authenticated user constraint from conversation write routes through the management service
• enforce conversation ownership before rename and message creation at the database boundary
• bind message edits to both the supplied conversation ID and its owner, closing the adjacent write path that previously ignored the conversation ID
• preserve existing superuser and trusted internal-call behavior, while returning the same 404-style response used by protected reads for unauthorized access

Closes #2292

Validation

• py/.venv/bin/pytest py/tests/unit/database/test_conversation_ownership.py py/tests/unit/app/test_routes.py -q --no-cov — 36 passed
• py/.venv/bin/ruff check py/core/main/api/v3/conversations_router.py py/core/main/services/management_service.py py/core/providers/database/conversations.py py/tests/unit/database/test_conversation_ownership.py — passed
• cd py && .venv/bin/python -m mypy --follow-imports=skip core/main/api/v3/conversations_router.py core/main/services/management_service.py core/providers/database/conversations.py — passed
• compileall and git diff --check — passed

Repository-wide baseline

pre-commit run --all-files was also exercised. The untouched origin/main baseline is not clean: mypy reports an existing UnstructuredClient | AsyncClient post union-attribute error in core/providers/ingestion/unstructured/base.py, reproduced from a clean detached origin/main worktree. The all-files formatter also changes existing unrelated files, so those changes were intentionally excluded from this focused security fix.

[yaodong-shen]

The Code Quality Checks and R2R Light Python Integration Test workflows are waiting for maintainer approval and created no jobs for this external-fork PR. The focused ownership regression suite, Ruff, and targeted mypy checks pass locally. Could a maintainer approve the workflow runs when convenient?