[I] Migrate Session to v4 API (invert MavenSession / Session) [maven]

["gnodet (via GitHub)" <gi...@apache.org>]

gnodet opened a new issue, #13462:
URL: https://github.com/apache/maven/issues/13462

   Part of #13460 — Migrate Maven internals from v3 to v4 API
   
   ## Context
   
   Currently the v4 `DefaultSession` wraps the v3 `MavenSession`. This means every v4 service that needs session state reaches through to the v3 object underneath. Inverting this is foundational for all later migration phases — every service needs a session.
   
   ## Goal
   
   Create a v4-native session implementation so `MavenSession` (v3) becomes a thin compat wrapper over the v4 session, instead of the other way around.
   
   ## Key files
   
   - **v4 API**: `api/maven-api-core/.../Session.java`, `ProtoSession.java`
   - **v4 impl**: `impl/maven-core/.../internal/impl/DefaultSession.java`
   - **v3 session**: `impl/maven-core/.../execution/MavenSession.java`
   - **Bridge**: `impl/maven-core/.../internal/impl/InternalMavenSession.java`
   
   ## Approach
   
   1. Create a v4-native session implementation in `impl/maven-impl` that holds its own state (projects, settings, repositories, properties, etc.)
   2. Make `MavenSession` (v3) a thin wrapper that delegates to the v4 session
   3. Update `DefaultMaven` to create the v4 session first, then wrap it for v3 callers
   4. `MavenSession` is imported in ~59 files — these don't all need to change at once, since `MavenSession` stays available
   
   ## Compatibility
   
   `MavenSession` stays available for plugin injection (`@Parameter`). Plugins calling `session.getCurrentProject()`, `session.getGoals()`, etc. continue to work unchanged.
   
   ## Dependencies
   
   - Ideally done after or alongside the Project Builder inversion (#13461), since the session holds projects
   
   ## Complexity
   
   High. `MavenSession` is a data holder used in 59+ files, but the inversion is conceptually straightforward since the interface doesn't change.
   
   _Claude Code on behalf of Guillaume Nodet_


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