Discarded generic method rows leave non-OnDispose event handlers on the class row's events #7373

Summary

A generic method row that is incompatible with a concrete instantiation can leave its builder-context handlers attached to the class row. The next compatible test then runs handlers registered for the discarded row.

Originally found while reviewing the now-closed PR #7328. Its replacement, open PR #7384 (delivering #7381), still returns the combined class/method event container from the incompatible-row branch via UnclaimedClassRow.Return. The old ReturnWithoutMethodRowAsync and SnapshotClassRowDisposeCallbacks names refer to the abandoned implementation, not current main or #7384.

Reproduction

Use a generic test method with a concrete instantiation, a class data source, and a method data source that calls RegisterForInitialization or subscribes OnTestStart. Yield an incompatible method row followed by a compatible row. The compatible test must not run the discarded row's handlers.

Acceptance criteria

• Preserve the class row's subscriptions when returning it for reuse.
• Drop the discarded method row's handlers from every event; raise its OnDispose callbacks exactly once without raising class-row cleanup early.
• Cover initialization and a non-disposal lifecycle event, plus disposal, in source-generated and reflection modes.
• Apply the fix to both eager and streaming builders while both exist; if TestBuilder.BuildTestsStreamingAsync is unused and duplicates the row-building loop #7327 has removed streaming, cover the remaining path.

Blocked by #7381, not the entire #7205 parent. No built-in data source subscribes the non-disposal handlers during a fetch, so those cases affect custom sources.