ci: split ci into a matrix job

[BjoernKarma]

CI: Refactor per-module jobs into a matrix build

What changed

The ci.yaml workflow previously defined 22 separate top-level jobs — one per module — each calling reusable-go-ci.yaml individually. This PR replaces that repetitive structure with:

• A single go_ci matrix job covering 19 modules (up to 6 running in parallel), with all per-module configuration encoded as matrix entries.
• Two dedicated direct jobs (common, common_server) for modules with non-standard flags (e.g. run_build_image: false for the library-only common module).
• One dedicated snapshotter job because it requires run_lint: false, which cannot be expressed in the shared matrix.
• A new ci_summary job that aggregates all results and acts as a single aggregation point for branch protection rules.

Why this makes sense

Before	After
22 individually written jobs	1 matrix + 3 special-case jobs
Adding a module = copy-pasting ~10 lines	Adding a module = 4–5 line matrix entry
All jobs visible as separate top-level status checks	Single CI Summary check aggregates everything
One failure could obscure others	fail-fast: false — all modules run, all failures visible

What is preserved

• All per-module flags (coverage_threshold, run_check_generated_files, lint_fail_on_issues, ko_build_path) are encoded in the matrix entries with the same values as before.
• Internal test-before-build ordering inside reusable-go-ci.yaml is unchanged — each call still runs tests → build image.
• Helm release path (common_server_helm) still depends on common_server and only runs on tag pushes.
• No behavior change on PR, push to main, tag, schedule, or workflow_dispatch triggers.

Action required after merge

If branch protection rules reference specific job names from the old workflow (e.g. "Approval / Tests & Coverage for approval"), update them to reference CI Summary as the single required status check.

[Copilot]

Pull request overview

This PR refactors the repository’s main CI workflow by replacing many per-module reusable-workflow jobs with a single matrix-driven job, while keeping a few special cases as dedicated jobs. It reduces repetition in .github/workflows/ci.yaml and introduces a single “CI Summary” job intended to be used for branch protection aggregation.

Changes:

• Replaced most per-module jobs with a go_ci matrix job (fail-fast disabled, max 6 in parallel) that calls reusable-go-ci.yaml with per-module parameters.
• Kept dedicated jobs for common, common_server, and snapshotter, plus the tag-only common_server_helm release job.
• Added a ci_summary job that summarizes results and fails the workflow if any required job fails (allowing common_server_helm to be skipped when not on tags).

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