[ejgallego]

This PR keeps Beam compatible with the VersionedTextDocumentIdentifier field rename in leanprover/lean4#16237, reverted by leanprover/lean4#16356. Positional construction and matching let the same source compile before and after the rename while preserving snapshot version checks and progress filtering.

• Keep the default toolchain and supported release window unchanged, and document the compatibility boundary and removal condition.
• Cover missing and null versions as raw JSON: progress notifications accept either, while snapshot-bound requests reject both and leave the document usable. Retain progress coverage for stale, current, and newer versions.
• Qualified on Linux x86-64 against the exact #16237 head (03b566a584a43808b6a4fd430b2842b5e087d3cc): tests/test-lsp.sh and the broker pending tests pass with the strengthened assertions. This exercises the reverted API change, which the current pinned-toolchain CI cannot cover.