fix(helm): support preinstalled JDBC drivers

[gutek9]

Description

This PR adds support for a new Helm JDBC delivery mode, jdbc.driverSource=preinstalled, for deployments where the JDBC driver is already baked into the final API and Gateway images.

In this mode, the chart no longer performs JDBC runtime provisioning: it does not create the JDBC initContainer, emptyDir, or plugin directory mount for API, Gateway, or the API upgrader job. The expected behavior is that the JDBC driver is already present in the standard plugin directory inside the custom runtime images.

The PR also updates the Helm documentation and adds regression coverage for the new mode, while preserving the existing behavior for auto, download, and image.

Additionally, it aligns Helm OpenShift security-context adaptation with global values support and applies the same adaptation to the API upgrader JDBC initContainer.

Additional context

Full Helm chart regression was executed with helm unittest -f 'tests/**/*.yaml' .
Result: 116/116 test suites passed, 608/608 tests passed
helm lint . also passed
For jdbc.driverSource=preinstalled, the expected driver locations are:
/opt/graviteeio-management-api/plugins/ext/repository-jdbc
/opt/graviteeio-gateway/plugins/ext/repository-jdbc
This mode disables JDBC runtime provisioning, but does not by itself enforce a read-only filesystem inside the running container

[gemini-code-assist]

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[sonarqubecloud]

Quality Gate passed for 'Gravitee.io APIM - Reporter'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud