fix(server): preserve in-memory SQLite across reconnects

[EmilienM]

🏗️ build-from-issue-agent

Summary

Preserve in-memory SQLite state when SQLx replaces the gateway's operational pooled connection. The SQLite adapter now retains a dedicated keepalive connection to the same named shared-cache database, preventing schema and object loss during sandbox cleanup.

Related Issue

Closes #3173

Changes

• Normalize anonymous in-memory SQLite URLs to a process-unique named database and enable shared cache.
• Retain a dedicated keepalive connection while leaving the operational pool serialized to one connection.
• Add deterministic regression coverage proving schema, existing objects, and subsequent writes survive operational connection replacement for both accepted anonymous URL forms.
• Document the in-memory SQLite lifetime invariant in architecture/gateway.md.

Deviations from Plan

• Expanded the regression and normalization to cover sqlite://?mode=memory after adversarial review identified that SQLx does not name that anonymous form automatically.

Testing

• mise run pre-commit passes — blocked by 17 pre-existing missing SPDX headers in unrelated .tekton/ and deploy/konflux/ files.
• cargo fmt --all -- --check
• Matched Rust 1.95 cargo clippy -p openshell-server --all-targets --features test-support -- -D warnings
• cargo test -p openshell-server --features test-support persistence::tests -- --nocapture (63 passed, 3 pre-existing ignored)
• Targeted explicit-delete and watcher-reconciliation compute tests
• The unrelated OIDC test that failed during the full concurrent suite passes in isolation
• E2E tests — not run because no e2e/ files changed and the existing Podman E2E uses file-backed SQLite, so it does not exercise this regression.

Tests added:

• Unit: persistence::tests::sqlite_in_memory_store_survives_pool_connection_replacement
• Integration: Existing compute deletion/reconciliation tests verified
• E2E: N/A; deterministic persistence regression covers the failure boundary directly

Adversarial review:

• Independent correctness/test review: clean after one confirmed alternate-URL finding was fixed and re-reviewed
• Independent architecture/maintainability review: clean on both passes

Checklist

• Follows Conventional Commits
• Commit is signed off (DCO)

Documentation updated:

• architecture/gateway.md: documented the in-memory SQLite keepalive invariant

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