test: harden fixtures and streamline CI

[gbaudrit]

Summary

This PR follows a complete review of the repository test suites and CI execution path.

• isolates every Testing web host in a single data directory and deletes owned SQLite/data-protection files on shutdown;
• treats a missing, empty, or whitespace-only Data:TestingDirectory as unconfigured and creates an owned temporary directory;
• preserves the existing non-Testing storage fallbacks under <ContentRoot>/.agentstration, even when Data:Directory is customized;
• lets fixtures provide Data:TestingDirectory when they own cleanup, including startup-failure scenarios;
• enables MSTest class-level parallelism for the Management suite while preserving the existing DoNotParallelize guard around environment-variable tests;
• clears only the SQLite pools owned by each fixture or test host, preventing parallel fixtures from disposing one another's live connections;
• removes the obsolete build.yml workflow, whose platform, AEP, and container jobs were fully duplicated by ci.yml;
• normalizes the mixed line endings in McpToolCatalogTests.cs in a dedicated commit.

Audit findings

• The repository has 11 platform test projects plus the separate AEP test project.
• The Management project was the critical path: 129 tests took about 3m39 serially.
• Web test hosts created identity, control-plane, work-plane, flow-plane, runtime, scheduler, and data-protection files in unrelated temporary locations. More than 123,000 agentstration-* entries had accumulated in %TEMP% on the review machine. Existing files were deliberately not deleted by this PR.
• A declarative-bootstrap startup-failure test could not reach the hosted cleanup service; its data is now owned and removed by the test fixture itself.
• Full validation exposed that process-wide SqliteConnection.ClearAllPools() calls were unsafe after enabling class-level parallelism. Cleanup is now scoped to the database files owned by each fixture.
• .github/workflows/build.yml repeated the complete platform test, AEP test, and container build already covered (with newer actions, formatting, timeouts, and Windows lifecycle coverage) by .github/workflows/ci.yml.
• No coverage threshold is currently enforced. This PR does not introduce a speculative threshold or new coverage dependency.
• Two live-provider tests remain intentionally optional and inconclusive unless LocalAI/llama.cpp endpoints are configured.

Compatibility

• Data:Directory continues to configure identity, scheduler, data-protection, and related general data paths outside Testing as before.
• The control, work, flow, and runtime plane fallbacks remain <ContentRoot>/.agentstration/*.db outside Testing, unless their explicit Data:*Path setting is provided.
• The unified per-host directory applies only to the Testing environment; this PR does not migrate or relocate production data.

Before / after

• Management suite: ~3m39 -> 2m00-2m32 across repeated runs.
• Complete platform suite: ~4m31 -> ~2m33.
• Owned test directories during the final complete run: 201 before / 201 after (no growth).
• CI no longer runs a second copy of the platform, AEP, and container jobs.

Validation

• dotnet format Agentstration.slnx --no-restore --verify-no-changes — passed
• dotnet build Agentstration.slnx --configuration Release --no-restore — 0 warnings, 0 errors
• dotnet test Agentstration.slnx --configuration Release --no-build — 539 total, 537 passed, 2 optional skipped, 0 failed
• Management suite repeated under class-level parallelism — 129/129 passed on every run, including two focused repetitions plus the final complete suite
• Testing-directory lifecycle coverage includes missing, empty, and whitespace-only configuration
• Final complete run temporary-directory count — 201 before, 201 after
• dotnet build aep/Aep.slnx --configuration Release --no-restore — 0 warnings, 0 errors
• dotnet test aep/Aep.slnx --configuration Release --no-build — 9/9 passed