FEAT: Full Code Coverage & Workflow Integration (#238)

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[AB#38622](https://sqlclientdrivers.visualstudio.com/c6d89619-62de-46a0-8b46-70b92a84d85e/_workitems/edit/38622)

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### Summary   
This pull request introduces significant improvements to the PR
validation pipeline by adding unified code coverage reporting for both
Python and C++ code, enhancing the build process for coverage
instrumentation, and optimizing the PR size label automation. The main
changes are grouped below:

**Pipeline Enhancements:**

* Added a new `CodeCoverageReport` job to
`eng/pipelines/pr-validation-pipeline.yml` that builds dependencies,
runs tests, generates unified coverage reports (Python + C++), and
publishes results in both HTML and Cobertura XML formats for Azure
DevOps. This includes setup for SQL Server, Python dependencies, and C++
coverage instrumentation.
* Commented out the old code coverage publishing task in the pipeline to
avoid duplicate or incomplete reporting, as the new job supersedes it.

**Build and Coverage Tooling:**

* Added a new script `generate_codecov.sh` to automate installation of
coverage tools, run tests, collect and merge Python/C++ coverage data,
and produce unified reports.
* Updated `mssql_python/pybind/build.sh` to support a coverage build
mode (`codecov` or `--coverage`), enabling Clang coverage
instrumentation for Linux builds and adjusting CMake configuration
accordingly.
[[1]](diffhunk://#diff-7cfcd9b5499a176c28ca4d028e9b2b060238ec79c4bea5b062d4b39929f9fb04R29-R35)
[[2]](diffhunk://#diff-7cfcd9b5499a176c28ca4d028e9b2b060238ec79c4bea5b062d4b39929f9fb04L50-R81)

**PR Automation Improvements:**

* Refined the PR size label automation in
`.github/workflows/pr-format-check.yml` to only update labels when
necessary, reducing unnecessary label churn and improving logging for
better traceability.

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Author: bewithgaurav