ci: align Azure Pipelines Markdown lint with GitHub workflow

## Problem

The required Azure Pipelines build runs:

```text
npx --userconfig eng/ci/.npmrc markdownlint-cli2 '**/*.md'
```

This floats the `markdownlint-cli2` version and relies on implicit configuration discovery, while the restored GitHub Lint workflow pins `markdownlint-cli2@0.17.2` and explicitly passes `--config .markdownlint.json`. The required and advisory CI paths can therefore disagree.

## Proposed change

- Pin the Azure Pipelines Markdown lint command to the repository-approved `markdownlint-cli2` version.
- Pass `--config .markdownlint.json` explicitly.
- Validate the `connectors-sdk.public` pipeline and the GitHub Markdown lint job against the same commit.

## Scope

Limit the change to the Markdown lint command in `eng/ci/templates/jobs/build-artifacts.yml` and any directly required package-lock or validation surface.

Follow-up from #252.

Start in eng/ci/templates/jobs/build-artifacts.yml and compare its Markdown lint command with the restored GitHub Lint workflow’s pinned version and explicit configuration. Validate the connectors-sdk.public pipeline and GitHub Markdown lint job against the same commit; done means both use the repository-approved version and .markdownlint.json.

Written by the indexing model from the issue text.