test(query): add cross-tenant isolation integration test for MCP query execution

[jsell-rh]

What & Why

The Per-Tenant Graph Routing requirement in specs/query/query-execution.spec.md
states:

"THEN it executes against the AGE graph named tenant_{tenant_id} for the
resolved tenant AND queries never cross tenant boundaries regardless of query
content."

The existing integration test suite (test_query_mcp.py) uses a shared test graph
(test_graph, see conftest.py line 71) rather than a tenant_{tenant_id}-named
graph. As a result, the cross-tenant isolation guarantee is never verified at the
integration level.

The unit tests in tests/unit/query/test_query_repository.py
(TestTenantGraphRouting) verify the behavior through mocks, but mock-based tests
cannot catch the real database behavior where:

• tenant_a could accidentally read tenant_b's data if the graph name resolution
  is broken
• The AGE SET GRAPH PATH or equivalent routing mechanism has a bug in production

This task adds an integration test that provisions two real AGE graphs with
tenant_ prefix names, writes distinct data to each, and verifies that queries
routed to tenant_a cannot see tenant_b's data.

Spec Requirements Satisfied

specs/query/query-execution.spec.md — Requirement: Per-Tenant Graph Routing:

• Scenario: Query routed to tenant graph — tenant_{tenant_id}
• Scenario: Tenant graph not found — rejected before reaching DB

Files Affected

• src/api/tests/integration/test_query_mcp.py
  — new test class TestCrossTenantIsolation with:
  • test_tenant_a_cannot_see_tenant_b_data: provision two AGE graphs
    (tenant_test_a, tenant_test_b), insert a unique node in each,
    query via QueryGraphRepository scoped to tenant_test_a, assert
    only tenant_a's node is returned
  • test_tenant_graph_not_found_raises_before_db: configure client with
    graph_name="tenant_nonexistent_xyz" (graph that doesn't exist),
    call execute_cypher, assert QueryExecutionError is raised and
    transaction() is never opened

Test Setup

The tests need a helper to create/drop AGE graphs:

def _create_age_graph(conn, name: str) -> None:
    conn.execute(text(f"SELECT ag_catalog.create_graph('{name}')"))

def _drop_age_graph(conn, name: str) -> None:
    conn.execute(text(f"SELECT ag_catalog.drop_graph('{name}', true)"))

Both graphs must be cleaned up in test teardown to avoid polluting other tests.

TDD Cycle

1. Write TestCrossTenantIsolation with the two test methods → RED (test infra
   needs the helper functions)
2. Implement helper functions → GREEN
3. Run: cd src/api && uv run pytest tests/integration/test_query_mcp.py::TestCrossTenantIsolation -v
4. Commit atomically

How to Verify

cd src/api
uv run pytest tests/integration/test_query_mcp.py::TestCrossTenantIsolation -v -m integration

Expected:

• test_tenant_a_cannot_see_tenant_b_data: only tenant_a's node returned ✓
• test_tenant_graph_not_found_raises_before_db: QueryExecutionError raised,
  no DB transaction opened ✓

Caveats

• AGE graph creation/deletion requires superuser or ag_catalog privileges — the
  test user must have CREATE GRAPH permission. Verify in the test DB setup.
• Tests must clean up their graphs in finally blocks to prevent leftover state.
• These tests are in the integration mark and require a running DB instance.

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Task: task-100
Spec: specs/query/query-execution.spec.md@dbcf0d7c2fa9c2456896ee20adbfdc8cc33090c2

Merge

The orchestrator will squash-merge this PR automatically
once all pipeline steps pass.

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This PR was created by hyperloop,
an AI agent orchestrator.