[Backport 4.0.x] Fix ToolchainPluginStrategy to detect inherited source levels from parent POMs - #13025

…rent POMs

* Fix ToolchainPluginStrategy to detect inherited source levels from parent POMs

When a project inherits --source/--release from a remote parent POM
(e.g. org.apache.sling:sling-parent setting --source 6), mvnup's
ToolchainPluginStrategy failed to detect it because detectSourceLevel()
only inspected the local POM XML DOM. This caused mvnup to skip adding
the toolchains plugin, leading to Maven 4 build failures.

Changes:
- Share the Maven 4 API Session across all strategy instances (DCL
  singleton) so the Session's RequestCache deduplicates effective model
  builds across PluginUpgradeStrategy, ToolchainPluginStrategy, and
  CompatibilityFixStrategy
- Route buildEffectiveModel() through InternalSession.request() to
  leverage the RequestCache
- Add effective model fallback in ToolchainPluginStrategy: when local
  POM has no source level, resolve the effective model to pick up
  inherited compiler configuration from parent POMs
- Add 8 new tests for effective model source level detection

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Extract MvnupSessionHolder with @provides @singleton Session producer

Move the Maven 4 API Session creation from AbstractUpgradeStrategy
into a dedicated MvnupSessionHolder class that uses @provides
@singleton to produce the Session as a DI-managed bean. The DI
container calls createSession() once and injects the singleton
Session into all strategy instances via @Inject @nAmed("mvnup")
field injection on the abstract base class.

This replaces the manual DCL singleton pattern with proper DI
lifecycle management. A static fallback in getSession() preserves
backward compatibility for unit tests that instantiate strategies
directly (without DI).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Address review: add groupId check in effective model plugin detection and fix Javadoc accuracy

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>