~_{Based on #41810 , which must be merged first.}~ => Done

Best reviewed commit by commit.

Effects of this PR:

1. Fixes #36666: see changes in expected behavior in `*DatasourceHealthCheckTest.java` for Agroal.
2. Fixes #43517: see changes in expected behavior in `*DatasourceHealthCheckTest.java` and `*ConfigUrlMissing*Test.java` for reactive SQL client extensions.
3. Improves error messages when an application attempts to statically inject or dynamically access a Datasource bean that is deactivated and/or has no URL set.
4. Introduces a few (arguably beneficial) breaking changes, see below.

Content to be added to the migration guide:

`````asciidoc

== Datasources

[[datasource-no-default-url]]
=== No default URL for Reactive SQL clients

In previous versions of Quarkus, when dev services were disabled, or when running in production mode, `quarkus.datasource.reactive.url`/`quarkus.datasource."datasource-name".reactive.url` would be implicitly set to an undocumented default targeting `localhost` and a DB-specific port.

Starting with this version of Quarkus, when dev services are disabled, or when running in production mode, `quarkus.datasource.reactive.url`/`quarkus.datasource."datasource-name".reactive.url` no longer has a default: if the property is not set, the corresponding datasource will be deactivated, leading to the behavior described in <<datasource-inactive-fail-fast>>.

If your application needs to target localhost for one of its datasources, set `quarkus.datasource.reactive.url`/`quarkus.datasource."datasource-name".reactive.url` explicitly, for example in `application.properties`.

[[datasource-no-health-check-if-no-url]]
=== Datasources without a URL no longer contribute a health check

In previous versions of Quarkus, when dev services were disabled, or when running in production mode, and when `quarkus.datasource.reactive.url`/`quarkus.datasource."datasource-name".reactive.url` was not set, a health check would be contributed for the corresponding datasource regardless, resulting in unreliable health checks that would either always succeed (JDBC datasource) or (almost) always fail (reactive datasources).

Starting with this version of Quarkus, datasources without a URL will no longer contribute a health check.

If your application needs a health check for a datasource, make sure to set `quarkus.datasource.jdbc.url`/`quarkus.datasource."datasource-name".jdbc.url`/`quarkus.datasource.reactive.url`/`quarkus.datasource."datasource-name".reactive.url` explicitly.

NOTE: Quarkus will still attempt to detect misconfiguration on application startup, but will report it differently, potentially earlier: see <<datasource-inactive-fail-fast>>.

[[datasource-inactive-fail-fast]]
=== Datasource usage fails fast if datasource is deactivated or has no URL set

In previous versions of Quarkus, when dev services were disabled, or when running in production mode, and when the datasource was deactivated (`quarkus.datasource.active=false`) or had no URL set, the application would be allowed to start successfully, but could fail much later upon first access to the datasource.

Starting with this version of Quarkus, datasources that are deactivated or do not have a URL will lead to a startup failure if Quarkus can detect they are used:

• Static CDI injection points involving the datasource (`@Inject DataSource ds` or `@Inject Pool pool`) will cause application startup to fail with an explicit, actionable message.
• Dynamic retrieval of the datasource (e.g. through `CDI.getBeanContainer()`/`Arc.instance()`, or by injecting an `Instance<DataSource>`) will not be detected on startup, but will cause an exception to be thrown with an explicit, actionable message.

The <<flyway-inactive-fail-fast,Flyway>> and <<liquibase-inactive-fail-fast,Liquibase>> extensions implement a similar behavior for their `Flyway`/`LiquibaseFactory` CDI beans.

If your application needs to inject a `DataSource`/`Flyway`/`LiquibaseFactory` bean for a datasource that can potentially be deactivated or have no URL, you may encounter a startup failure similar to this:

```
io.quarkus.arc.InactiveBeanException: Bean is not active: SYNTHETIC bean [class=io.agroal.api.AgroalDataSource, id=sqqLi56D50iCdXmOjyjPSAxbLu0]
Reason: Datasource '<default>' was deactivated automatically because its URL is not set. To activate the datasource, set configuration property 'quarkus.datasource.jdbc.url'. Refer to https://quarkus.io/guides/datasource for guidance.
To avoid this exception while keeping the bean inactive:

• Configure all extensions consuming this bean as inactive as well, if they allow it, e.g. 'quarkus.someextension.active=false'
• Make sure that custom code only accesses this bean if it is active
• Inject the bean with 'Instance<io.agroal.api.AgroalDataSource>' instead of 'io.agroal.api.AgroalDataSource'
  This bean is injected into:
  -io.quarkus.agroal.test.ConfigUrlMissingDefaultDatasourceStaticInjectionTest$MyBean#ds
  at io.agroal.api.AgroalDataSource_sqqLi56D50iCdXmOjyjPSAxbLu0_Synthetic_Bean.doCreate(Unknown Source)
  [...]
  ```

To avoid this failure, follow the instructions in the exception message.
In particular, consider injecting the problematic bean as an `InjectableInstance` (for example `@Inject InjectableInstance<DataSource> ds`) instead.
You can check whether that bean is active using `.getHandle().getBean().isActive().result()` on the `InjectableInstance`, and retrieve the bean instance using `InjectableInstance#get()`.
See https://quarkus.io/guides/datasource#datasource-active for an example of such a setup for `DataSource` beans – the idea would be similar for a `Flyway`/`LiquibaseFactory` beans.

== Flyway

[[flyway-inactive-inactive-fail-fast]]
=== Flyway usage fails fast if datasource is deactivated or has no URL set

Starting with this version of Quarkus, datasources that are deactivated or have no URL set will lead to a startup failure if Quarkus can detect a `Flyway` bean is used for that datasource.

See the <<datasource-inactive-fail-fast,datasources,similar migration guide entry for datasources>> for more information.

== Liquibase

[[liquibase-inactive-if-no-url]]
=== Liquibase usage fails fast if datasource is deactivated or has no URL set

Starting with this version of Quarkus, datasources that are deactivated or have no URL set will lead to a startup failure if Quarkus can detect a `LiquibaseFactory` bean is used for that datasource.

See the <<datasource-inactive-fail-fast,datasources,similar migration guide entry for datasources>> for more information.
`````