CI: rename everything-mcp-diff.yml to mcp-interface-diff.yml, staged across a milestone merge [Step 1] #5463

Problem

.github/workflows/everything-mcp-diff.yml diffs every server's MCP interface since #5459 (#5428), but its file name still says everything. #5459 renamed it to mcp-interface-diff.yml at first, and backed that out after review: GitHub dispatches a workflow only when its file exists on the default branch (main). Both version workflows (version-packages.yml, prepare-python-release.yml) dispatch it by file name from v2/main, so a one-step rename on v2/main breaks their dispatch until the next milestone merge, and the first version PR after it gets no Interface diff (dispatched) status.

Expected behavior

The workflow file is named for what it does (mcp-interface-diff.yml), and version PRs keep their dispatched diff throughout the transition.

Notes for the implementation

Stage the rename across a milestone merge:

1. Add mcp-interface-diff.yml beside the old file, with the callers unchanged, and ship it to main in a milestone merge. Make sure only one of the two runs on push / pull_request (for example, the new file is workflow_dispatch-only until step 2).
2. After that merge, switch the callers to the new file and delete the old one: CI: rename everything-mcp-diff.yml to mcp-interface-diff.yml, staged across a milestone merge [Step 2] #5527 (Step 2).

Related: #5428, #5459, #5527 (Step 2).