[gha] Improve summary

[AlexanderDokuchaev]

Changes

Combine summary in one note, instead of multiple notes for each sub job from matrix

Tests

https://github.com/openvinotoolkit/nncf/actions/runs/21748943693
https://github.com/openvinotoolkit/nncf/actions/runs/21748943736

[Copilot]

Pull request overview

This PR improves GitHub Actions test reporting by uploading per-matrix pytest XML results as artifacts and generating a single consolidated Markdown summary per workflow/job group.

Changes:

• Upload pytest-results.xml as an artifact for each matrix shard instead of writing multiple step summaries.
• Add dedicated “Summary” jobs that download all artifacts and generate one combined Markdown table.
• Update pytest_md_summary.py to support multiple XML inputs and output a combined table.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
.github/workflows/examples.yml	Upload shard-level pytest XML artifacts and add aggregate summary jobs (CPU/CUDA/Windows).
.github/workflows/conformance_weight_compression.yml	Upload shard-level pytest XML artifacts and add an aggregate summary job.
.github/scripts/pytest_md_summary.py	Support parsing multiple XML files / glob patterns and emit a consolidated Markdown summary.

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