mongodb: vendor chart via git subtree (alternative to #2420)

[delthas]

Draft — alternative implementation of ZENKO-5281; the team will pick this or #2420 and close the other.

Summary

Switch solution-base/mongodb/Makefile from helm fetch against charts.bitnami.com to vendoring the chart sources from the bitnami/charts GitHub repository via git subtree, mirroring the pattern that ZENKO-5110 already established for vendored Bitnami images in solution-base/images/Makefile. All 11 local modifications previously expressed as .patch files now live as ordinary git commits on top of the subtree merge.

Why this exists alongside #2420

The first PR for ZENKO-5281 (#2420) implemented the same goal via curl | tar of the GitHub release tarball. PR reviewer @francoisferrand suggested git subtree instead, both to match the solution-base/images/ precedent and to make future upstream merges produce real three-way merge conflicts rather than silent patch overwrites. This PR is that alternative; we'll compare locally and merge whichever the team prefers.

What changes

• solution-base/mongodb/Makefile — full rewrite. New targets create-remote, fetch-remote, update-vendor-branch, vendor-sync, deps. The single maintainer command is make -C solution-base/mongodb vendor-sync (~4 minutes — fetches bitnami/charts history, runs git subtree split, merges as squash, re-resolves common).
• solution-base/mongodb/charts/mongodb-sharded/ — re-introduced as a git subtree --squash of upstream tag mongodb-sharded/9.4.4. The common library chart is resolved via helm dependency build and extracted into charts/common/ so solution-base/build.sh's helm template finds it as a sub-chart.
• 11 patch files → 11 individual commits. Each one's effect is reproduced verbatim; the commit message references the original patch filename for traceability.
• solution-base/mongodb/patches/ deleted entirely.
• solution-base/mongodb/how_to_upgrade.md rewritten to describe the new flow.
• .github/workflows/end2end.yaml — check-mongo-patches job removed (it verified make fetch + make patch produced no drift; with patches gone there is nothing to verify, and conflicts now surface as ordinary git merge conflicts during make vendor-sync).
• .github/copilot-instructions.md — the Chart upgrade-path row now points at Makefile + make vendor-sync instead of patches/ + make patch.

What does NOT change

• Pinned chart version stays at 9.4.4 — pure infrastructure switch. ZENKO-5276 becomes a CHART_VERSION bump + make vendor-sync whose merge brings in upstream's pgrep -x liveness probe fix.
• solution-base/deps.yaml is untouched — image references are independent and addressed under ZENKO-5110.
• solution-base/build.sh is untouched — it expects charts/common/ as an extracted directory, which the new Makefile preserves.

Verification

• helm template against this branch and against mongodb: vendor chart via git subtree from bitnami/charts #2420's branch produce identical manifests (modulo helm-generated random Secret values per invocation).
• make -C solution-base/mongodb vendor-sync is idempotent against mongodb-sharded/9.4.4: re-running on a clean tree produces no diff.
• Each of the 17 commits is reviewable in isolation.

Costs

• Maintainer-side make vendor-sync is a ~4-minute coffee break (subtree split walks 36 599 upstream commits).
• One-time .git growth of ~41 MB in the maintainer's clone from the synthesized subdir commits; not pushed to origin.
• bitnami-charts remote needs to be configured (Makefile does this automatically on first run).

Issue: ZENKO-5281

[bert-e]

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[delthas]

Closing — the subtree implementation has been force-pushed onto #2420, which keeps the original review history. The branch this PR points at will be deleted shortly.