Fix/scope the Scalar filter to the Scalar starters

[awinishere]

Problem

springdoc-openapi-starter-common registers an AutoConfigurationImportFilter that drops Scalar's
own auto-configurations. That module is a dependency of every starter, so applications using the
Swagger UI or the API-only starter lost the standalone Scalar integration too, with no Springdoc
Scalar controller replacing it: the configured Scalar path answered 404.

Change

The registration moves into the META-INF/spring.factories of each Springdoc Scalar starter, so it
applies exactly where a Springdoc Scalar controller replaces the standalone one. This is the first
option suggested in #3387. No logic changed ScalarDisableAutoConfiguration stays in
starter-common and still excludes both auto-configurations (the file move shows as R100,
byte-identical).

Both auto-configurations stay excluded together on purpose: Scalar's reactive auto-configuration has
no @ConditionalOnWebApplication, and both sides declare the bean scalarWebMvcController on
${scalar.path}. Letting the standalone one survive next to a Springdoc Scalar starter fails startup:

BeanDefinitionOverrideException: Invalid bean definition with name 'scalarWebMvcController' ...
already defined in [com/scalar/maven/webmvc/ScalarWebMvcAutoConfiguration.class]

Verified end to end

Same Boot 4.1.0 app (com.scalar.maven:scalar-webmvc:0.6.80, scalar.path=/docs), only the springdoc
dependency / filter registration changes:

springdoc	/docs
3.1.0 (released)	404 — the reported bug
this branch	200, served by com.scalar.maven.webmvc.ScalarWebMvcController
this branch + filter registered manually by the app	404 — confirms the filter was the cause
starter-webmvc-scalar + standalone auto-config force-@Imported	startup fails (the #3084 guard is real)
starter-webmvc-scalar + standalone (normal)	200, served by the Springdoc controller ("url":"http://localhost:8098/v3/api-docs")

/v3/api-docs, /swagger-ui/index.html and business endpoints stayed 200 in every scenario.

Behaviour change

The standalone integration is enabled by default (scalar.enabled uses matchIfMissing=true);
opt out with scalar.enabled=false. Also worth documenting: the standalone page needs scalar.url,
otherwise it renders a demo spec hosted on registry.scalar.com with telemetry: true, so the
visitor's browser makes a third-party request. The Springdoc controller derives the URL from the
request and has no such default.

Tests

New: ScalarDisableAutoConfigurationTest (starter-common), ScalarStarterRegistersTheDisableFilterTest
(both starters), SpringDocApp20Test (webmvc-scalar, the standalone integration must not survive next
to the Springdoc controller)

Fixes #3387

[Mattias-Sehlstedt]

To my knowledge the test change should not be necessary, since the date change which I did in d78abd9 was to remove springdoc pinning the type to string. We instead cede the resolving to swagger-core, which resolves it to partial-time by default. The type should only become time-local based upon an opt-in, which is described in swagger-api/swagger-core#5184.

[awinishere]

To my knowledge the test change should not be necessary, since the date change which I did in d78abd9 was to remove springdoc pinning the type to string. We instead cede the resolving to swagger-core, which resolves it to partial-time by default. The type should only become time-local based upon an opt-in, which is described in swagger-api/swagger-core#5184.

That makes sense my apologies for the extra change. the branch is now just the Scalar fix.
Thanks for the review.