test: add MaintenanceRequestsController integration tests (Story 21.1)

[daveharmswebdev]

Summary

• Backfills 27 WebApplicationFactory-based integration tests covering all 4 endpoints of MaintenanceRequestsController (Epic 21, Story 21.1 — closes test-coverage gap identified in issue Testing pyramid gaps: integration + E2E coverage for newer features #371)
• Extends PropertyManagerWebApplicationFactory additively with CreateTenantUserInAccountAsync and CreatePropertyInAccountAsync — existing helpers unchanged (verified against 558 pre-existing Api.Tests)
• Zero production code changes — ACs were reconciled against the shipped controller contract and every delta is documented in the story's Dev Notes

Coverage

• POST /maintenance-requests (create, validation, tenant→property linkage via JWT claim)
• GET /maintenance-requests (admin account-scoped, tenant property-scoped, status filter, pagination)
• GET /maintenance-requests/{id} (access control — returns 404, not 403, to avoid existence disclosure)
• GET /tenant/property/maintenance-requests (tenant property-scoped list)

Test plan

• dotnet test — 1845/1845 passing (558 Api + 1189 Application + 98 Infrastructure)
• npm test — 2751/2751 frontend unit tests passing
• npx playwright test — 212/213 (1 pre-existing Reports-flow flake, unrelated)
• Backend + frontend builds clean (0 warnings, 0 errors)
• Adversarial review: tests assert meaningful behavior (404 specificity, CreatedAt ordering, soft-delete filter, tenant isolation)

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