[AaronFeledy]

Summary

Adds opt-in MySQL 8.4 via database.type: mysql and database.version: 8.4. MariaDB remains the default, including existing version and ARM paths. Invalid combinations fail with an actionable error.

• Uses the existing MySQL 8.4 service with Pantheon credentials and a dedicated server config: utf8mb4, utf8mb4_0900_ai_ci, and READ-COMMITTED.
• Removes the global appserver TLS exception. Pull/push probe each local client and apply the MariaDB verification option only to local MySQL 8.4 connections. Oracle clients do not receive MariaDB flags; remote commands and TLS arguments from Terminus remain unchanged.
• Propagates the effective database service type, including Landofile overrides, to pull/push/switch.
• Adds --no-tablespaces only to local MySQL 8.4 dumps.
• Stages source dumps before importing. Failed source dumps are not imported or pushed; pull can retry through its remote dump fallback. Import and pipeline failures stop the sync. Staging requires temporary disk space.
• Does not upgrade appserver images or migrate volumes. Switching engines requires recreating local database storage and importing data again.

Fixes #374

Verification

• npm test: 84 passing, 1 existing pending, including local/remote flag separation, client capabilities, service overrides, fallback, quoting, and failure propagation.
• All scripts/*.sh pass bash -n; git diff --check passes.
• Real Docker: MySQL 8.4.5 with Pantheon appserver 8.4-5 / MariaDB 11.4.9; query, import, full dump/restore, checksummed 300 rows, Unicode/binary values, view/trigger, and native failures.
• Actual MariaDB 10.6.20, MariaDB 10.11.11, and Oracle 8.4.5 clients query/dump successfully. Oracle restores the MariaDB-generated dump.
• Actual pull/push scripts exercised against disposable local schemas, including fallback, failure, and recovery. Terminus/auth were mocked, so this is not a real Pantheon-hosted sync.
• Slop pass: no wording cuts. Local Cursor Bugbot review of bd2254a: no bugs found.

Leia integration coverage

• Added local-only examples/pantheon-mysql84 and wired both 3-stable and 3-edge CI cells, with explicit MySQL plugin 1.6.0 provisioning.
• Actual Leia run: 8 passing (47s) on core 3.26.9, PHP plugin 1.12.0, pristine MySQL plugin 1.6.0, and this Pantheon checkout.
• Checks real server MySQL 8.4/defaults, non-root PDO connectivity and exact Unicode rows, local helper dump/restore, and recipe db-export/db-import round-trip. Teardown affects only this example; no global poweroff.
• Independent Astra review approved the implementation. Its misleading release-prerequisite documentation finding was corrected in 16d2605.
• Local success is not a claim that both GitHub CI matrix cells have passed; real remote Pantheon synchronization remains unverified.

Limits and follow-up

• No disposable Pantheon environment was supplied; real remote synchronization remains unverified.
• Drush and arbitrary appserver commands are not covered by this local sync workaround. Selecting MySQL does not upgrade old client binaries or fix all authentication/collation compatibility.
• Shared PHP client provisioning through updated Pantheon images is a separate change. Current generation-5 images lack the bundled Oracle clients.

Bare minimum self-checks

• I've updated this PR with the latest code from main
• I've done a cursory QA pass of my code locally
• I've ensured all automated status check and tests pass
• I've connected this PR to an issue

Pieces of flare

• I've written a unit or functional test for my code
• I've updated relevant documentation it my code changes it
• I've updated this repo's README if my code changes it
• I've updated this repo's CHANGELOG with my change unless its a trivial change (like updating a typo in the docs)

Finally

• I've requested a review with relevant people

---

Note

Medium Risk
Changes core database sync (pull/push) and engine selection; mistakes could corrupt local DBs or break remote sync, though behavior is heavily tested and scoped to MySQL 8.4 compatibility paths.

Overview
Adds opt-in MySQL 8.4 when pantheon.yml sets database.type: mysql and database.version: 8.4; MariaDB stays the default, with validation errors for invalid combinations. The recipe selects mysql:8.4, applies a new mysql84.cnf (utf8mb4, utf8mb4_0900_ai_ci, READ-COMMITTED), and passes the effective service type—including Landofile overrides—to lando pull, push, and switch via LANDO_DB_SERVICE_TYPE.

Pull/push now finish dumps to a temp file before import (pipefail), retry pull’s remote mysqldump fallback without importing partial SQL, and use pantheon-local-database.sh to add MariaDB-only local flags (--ssl-verify-server-cert=0, --no-tablespaces on dump) only for local MySQL 8.4; Terminus remote TLS/commands are unchanged.

Docs/CHANGELOG, a local-only Leia example (examples/pantheon-mysql84), CI matrix + explicit @lando/mysql@1.6.0 install, and unit/integration tests cover selection, client probing, and failure propagation.

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