feat(testing): add Markdown formatter for test reports

[diiviikk5]

Summary

Add a MarkdownFormatter to the mofa-testing report module.

This PR extends the existing formatter path alongside JsonFormatter and TextFormatter with a Markdown-oriented output that is easier to use in PR comments, docs, and CI summaries.

Closes #1546.

What Changed

• added MarkdownFormatter in tests/src/report/format.rs
• exported MarkdownFormatter through the report module and crate root
• added focused report tests for:
  • summary and results table rendering
  • escaping | and multiline values inside cells
  • empty report output

Why

mofa-testing already supports JSON and plain text output, but Markdown is a useful human-readable middle ground for:

• PR summaries
• contributor-facing examples
• proposal/demo artifacts
• CI-generated readable summaries

This keeps the change small and directly aligned with the report layer already in the crate.

Testing

Ran:

cargo test -p mofa-testing --test report_tests

Result:

• report_tests: 27 passed, 0 failed

Scope

This PR is intentionally limited to report formatting.
It does not introduce:

• benchmark snapshot rendering
• CI comment posting
• HTML output
• changes to the TestReport data model beyond formatter exposure