ci(test): publish functional coverage reports

[gbaudrit]

Summary

Publishes a consolidated functional code coverage report in GitHub Actions for the deterministic Fast and Integration lanes without using an external coverage service. Closes #349.

Changes

• Collects Microsoft Testing Platform coverage from both functional lanes in Cobertura format while preserving their test-count and parallelism safeguards.
• Adds a shared coverage configuration that includes product sources and excludes test assemblies, Performance workloads, generated code, and non-product source paths.
• Pins ReportGenerator 5.5.11 as a repository-local .NET tool and uses it to merge module reports into consolidated Cobertura and browsable HTML output.
• Publishes consistent line and branch totals to the GitHub Actions job summary and uploads raw plus consolidated reports as the functional-code-coverage artifact.
• Adds a single reproducible local command and documents the report-only policy in the testing guide and agent instructions.

Validation

• dotnet tool restore succeeded with the pinned ReportGenerator 5.5.11 tool.
• dotnet build Agentstration.slnx --configuration Release --no-restore succeeded with 0 warnings and 0 errors.
• Coverage Fast lane: 139 passed, 0 failed, 0 skipped.
• Coverage Integration lane: 689 passed, 0 failed, 3 optional provider/storage integrations skipped; 692 total.
• Nineteen per-module Cobertura files merged successfully into a 4.48 MiB consolidated Cobertura report and a browsable HTML report.
• GitHub Actions consolidated baseline: 15,784 of 24,348 lines covered (64.83%) and 7,145 of 11,434 branches covered (62.49%). No threshold is enforced.
• Report validation confirmed that test assemblies, Performance tests, generated sources, and test source paths are absent.
• All pull request checks passed, including build-and-test, CodeQL, dependency review, container, PostgreSQL storage, and Windows host lifecycle.
• Desktop and mobile smoke testing not applicable; this change affects test execution and CI reporting only.

Breaking changes

None.