fix: scope unscoped proposal and test-run lists to the caller's projects (#482)

[JabbaKadabra]

Summary

GET /api/proposals and GET /api/test-runs were the last two list endpoints reading GetAccessibleProjectIdsAsync directly and demanding a filter — a non-admin sending neither projectId nor agentId got a successful 200 with an empty list instead of the rows of the projects they can see. The eight controllers named in the issue were already fixed by #484 (which closed no issue, so #482 stayed open); these two were not. Both now resolve a scope through ResolveListScopeAsync like every other list endpoint and apply it inside the query.

Closes #482

Changes

• ProposalsController — replaces the CanListAsync "return nothing" helper with the ListScopeAsync shape TestRunGroupsController already uses. Side effect worth noting: a named agentId outside the requested projectId no longer bypasses that project filter.
• TestRunsController — an unfiltered list now resolves ResolveListScopeAsync(requestedProjectId: null) instead of short-circuiting every non-admin to an empty page.
• IOptimizationProposalRepository.GetByProjectsAsync and ITestRunRepository.GetByProjectsPagedAsync — set-aware lookups so the scope is an IN predicate in SQL and the page is computed over the union, not merged from per-project pages. GetByProjectAsync now delegates to the set overload.
• Regression tests at both layers: controller tests for own-project / multi-project-union / no-accessible-projects, and storage tests for the new repository methods (including the includeSystem behaviour for runs).
• docs/architecture.md — documents the related-entity (agentId/suiteId) narrowing shape and the no-projectId-parameter case; the "no controller re-implements the check" claim is now actually true.
• manual/admin/providers-and-api-keys.md + CHANGELOG.md — extend the existing Unscoped list endpoints return an empty page instead of the caller's accessible rows for non-admins #482 entry rather than duplicating it.

Verification

• build: dotnet build Proxytrace.sln — 24 projects, 0 errors, 0 warnings
• tests: full backend suite (dotnet test Proxytrace.sln) — 2,797 passed, 0 failed, 6 skipped. Ran the full suite because the change alters two shared repository interfaces. The four new controller tests were confirmed to fail with the controller fix reverted.
• docs/changelog: docs/architecture.md, manual/admin/providers-and-api-keys.md (npm run docs:build green), CHANGELOG.md [Unreleased]. No UI strings, so no i18n extract.

Not done

No perf metric was added. The high-volume path for multi-project scope is already covered by agentCallsListByProjects; TestRunEntity paging has never had perf coverage (GetAllPagedAsync/GetByAgentPagedAsync included), and the new query is the same join shape as the existing TestRunGroupRepository.GetByProjectsPagedAsync with IN instead of =. Adding one would need the perf seeder to build run chains at scale — worth doing, but a separate change.

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