Remove the deprecated DoclingServeApiBuilderFactory SPI and related types

The work is in the docling-serve-api and docling-serve-client modules. Start by reviewing the deprecation in issue #696 and the serve-api-provider-migration.md guide. The checklist names specific files to delete (e.g., DoclingServeApiBuilderFactory, LegacyProviderAdapter) and tests to update. 'Done' means all deprecated types are removed, tests pass, and documentation is updated.

#696 replaced the DoclingServeApiBuilderFactory SPI with DoclingServeApiProvider and deprecated the old types for removal in 0.7.0, keeping a fallback so existing implementations keep working. This issue tracks removing them.

Before removing

• At least one minor release has shipped with the deprecation, so no earlier than 0.8.0.
• Known third-party implementations, such as the Quarkus integration, have moved to DoclingServeApiProvider.

docling-serve-api

• Delete ai.docling.serve.api.spi.DoclingServeApiBuilderFactory, and its uses clause in module-info.java.
• Delete DoclingServeApi.DoclingApiBuilder and DoclingServeApi.toBuilder().
• Delete LegacyProviderAdapter and LegacyProviderAdapterTests.
• DoclingServeApiProviders: remove the legacy fallback, the warning logged when both SPIs are present, and the "(or of the deprecated …)" part of the no-provider message. Update DoclingServeApiProvidersTests and DoclingServeApiTests.
• Update the Javadoc of DoclingServeApi.builder(), which describes the fallback.
• TestApis.StubDoclingServeApi: remove its toBuilder() override.

docling-serve-client

• Delete DoclingServeClientBuilderFactory and DoclingServeClientBuilderFactoryTests. The Jackson detection lives in DoclingServeClient.builder().
• DoclingServeClientBuilder: drop implements DoclingApiBuilder<C, B> and the @Override annotations of its setters. The setters themselves stay.
• Keep DoclingServeJackson2Client.toBuilder() and DoclingServeJackson3Client.toBuilder(), which return the concrete builder and aren't deprecated, but drop their @Override.
• Tests calling DoclingServeApi.toBuilder() (DoclingServeJackson2ClientTests, DoclingServeJackson3ClientTests, AbstractDoclingServeClientAsyncExecutorTests, which show [removal] warnings today) should use config().toBuilder() or the concrete client type.
• AbstractDoclingServeClientConfigTests.toBuilderKeepsEverySetting and toBuilderKeepsFollowingRedirects call toBuilder() on a DoclingServeClient, which resolves to the deprecated DoclingServeApi.toBuilder() (warnings suppressed). They test the copy constructor, so they must call toBuilder() on the concrete client type, not config().toBuilder().

Both modules

• Remove every @SuppressWarnings("removal") that only existed for these types, in main and test code.

Docs

• serve-api-provider-migration.md: remove the Compatibility section on the fallback, or retire the page if it's no longer useful.
• serve-api.md: remove the "Migrating from DoclingServeApiBuilderFactory" subsection.
• CLAUDE.md: remove the mention of the fallback.
• whats-new.md: add a "Breaking" entry for the removal.