feat(AGX1-277): enforce parent-task authz on GET /messages/{message_id}

[harvhan]

Linear Issue

AGX1-277 — Parent epic: AGX1-264.

Summary

Guard GET /messages/{message_id} (previously unauthorized) by resolving the message to its task_id and requiring task.read on the parent task. Wires this through the existing TaskChildResourceType pattern — message joins event and state so the standard DAuthorizedId(...) machinery handles it.

Collapses task-child auth denials to 404 (ItemDoesNotExist) across path-id and query-id surfaces so callers cannot probe cross-tenant existence by comparing 403 vs 404.

Corrects the misleading TaskMessage | None return type on get_message (use case + route) — the underlying service already raised ItemDoesNotExist; the None branch was dead code.

Changes

• src/api/schemas/authorization_types.py — add message to TaskChildResourceType.
• src/utils/authorization_shortcuts.py — thread DTaskMessageRepository through _get_parent_task_id and inject message_repository into the _ensure_authorized_id / _ensure_authorized_query deps. Wrap the task-child check in try/except AuthorizationError → ItemDoesNotExist to collapse denial to 404 on both surfaces.
• src/api/routes/messages.py — add DAuthorizedId(TaskChildResourceType.message, AuthorizedOperationType.read, ...) to get_message; drop the dead else None branch and tighten the return type.
• src/domain/use_cases/messages_use_case.py — fix get_message return annotation from TaskMessageEntity | None to TaskMessageEntity.
• tests/integration/api/messages/test_messages_authz_api.py (new) — integration tests covering the auth matrix.

Behavior change beyond the ticket

The 404 collapse extends to event and state child routes as well, since the TaskChildResourceType branch is shared. The cross-tenant-leak argument is the same for those resources. Aligns with the direction AGX1-275 takes more broadly.

Out of scope

• POST/PUT message routes still surface 403 on deny. They use DAuthorizedBodyId(task, execute) and will be rewritten to task.update + 404-collapse by AGX1-275.
• RPC message/send similarly handled by AGX1-275.
• The agent-API-key path (request.state.agent_identity) still bypasses authz via AuthorizationService._bypass(). Tracked separately in AGX1-305.

Coordination

Will conflict with AGX1-275 (#249) on the _ensure_authorized_id / _ensure_authorized_query bodies — that PR introduces a shared _check_task_or_collapse_to_404 helper in src/utils/task_authorization.py that replaces the inline try/except wraps in this PR. Trivial 3-way merge regardless of merge order; the inline wraps in this PR get replaced by the canonical helper call when the two land together.

Test Plan

• New: tests/integration/api/messages/test_messages_authz_api.py — 4/4 passing (authorized GET → 200, denied GET → 404 via collapse, non-existent message → 404 with explicit assertion that no authz call fires, authorized list happy-path).
• Regression: tests/integration/api/messages/test_messages_api.py — 19/19 passing.
• Regression: tests/integration/api/events/ — 3/3 passing (exercises the changed _get_parent_task_id signature on the event path).
• Regression: tests/integration/api/states/ — 7/7 passing (same, for state).
• Regression: tests/integration/api/agents/test_agents_auth_api.py — 3/3 passing (exercises the changed _ensure_authorized_id/_ensure_authorized_query factories on a direct-resource path).
• Ruff lint + format clean across all changed files.