Fix duplicate menu entries and inconsistent Test Plan naming during merge.

[ruthst00]

Description

Both bugs from issue #6633 have been fixed across four files:

Bug 1 — Inconsistent Test Plan name after merge (3 files changed):

• JMeterTreeModel.addSubTree() now has an overload that accepts a merging boolean. When merging=true, the userObject.setName(item.getName()) call is skipped, so the current plan's name is always preserved regardless of which node was selected at the time of the merge.
• GuiPackage.addSubTree() gained a matching addSubTree(HashTree, boolean merging) overload that threads the flag down to the tree model.
• Load.insertLoadedTree() now calls guiInstance.addSubTree(tree, merging) instead of the no-arg form, so the flag flows end-to-end from the user action all the way to the model.

Bug 2 — Duplicate Open / Merge / Save Selection As menu entries (1 file changed):

• MenuFactory.addFileMenu() no longer adds Open, Merge, or Save Selection As items to the node popup / Edit menu. Those three are file-level actions that already live in the File menu bar; having them in the right-click popup (which is mirrored into the Edit menu) was the sole source of the duplication. A comment was added explaining the intentional omission.

Motivation and Context

Fixes #6633

There is inconsistent Test Plan naming and duplicate menu entries after merge:

• If Test Plan node is selected: merged file contents are added, but the Test Plan name remains from the current plan (e.g., Test Plan A).
• If a Thread Group node is selected: merged file contents are added, but the Test Plan name changes to the merged file’s name (e.g., Test Plan B).
• Duplicate menu entries exist: File → Merge… and Edit → Merge…, File → Open… and Edit → Open…, File → Save Selection As… and Edit → Save Selection As….
• These perform the same action, which may confuse users.

How Has This Been Tested?

Two new test files were added:

• src/core/…/gui/tree/JMeterTreeModelMergeTest.java — 3 pure unit tests (no GUI required):
• src/dist-check/…/gui/util/MenuFactoryFileMenuTest.java — 4 integration tests (placed in dist-check because MenuFactory's static initializer requires the full JMeter classpath)

Generated with Claude Sonnet via Cline

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.