test: add cross-tenant isolation tests supporting upstream_header

[rhdedgar]

What does this PR do?

• Adds 6 tests to test_multi_tenant_files_e2e.py and 5 tests to test_multi_tenant_responses_e2e.py, extending the pattern from the existing conversations tests in test_multi_tenant_e2e.py to cover files and responses.
• Tests exercise the full middleware stack (AuthenticationMiddleware -> TenancyMiddleware -> ProviderDataMiddleware -> AuthorizedSqlStore) with upstream_header auth, verifying tenant isolation at the HTTP level.

Currently, conversations had HTTP-level isolation tests, but files and responses were only tested at lower layers like AuthorizedSqlStore unit tests, and integration tests with Bearer tokens. These tests close that gap by verifying that cross-tenant reads, deletes, and list operations return 404/empty results through the actual HTTP path.

Closes RHAIENG-6646

Test Plan