Aggregate code coverage reports to identity total code coverage

[aaronburtle]

Combined code coverage across pipelines

Code Coverage in Azure DevOps shows as relatively low for any single
pipeline because DAB's tests are split across six independent pipelines (unit +
MsSql, PostgreSql, MySql, DwSql, CosmosDb) and nothing merges their results. The
unit pipeline covers Config/Mcp/GraphQL/validators; the DB pipelines cover the
Core SQL engines. They are complementary, so each in isolation looks low while
the true union is significantly higher.

Add an aggregation step that unions the coverage every pipeline already produces
and publishes a single combined number for visibility.