Require workspace refresh support before enabling pull diagnostics

[lcroberts]

Only negotiate and enable pull diagnostics (textDocument/diagnostic) when the client explicitly advertises workspace.diagnostic.refreshSupport. Clients that advertise pull diagnostics without refresh support (e.g. Neovim ≤ 0.11)
now fall back to standard push diagnostics (textDocument/publishDiagnostics).

I originally noticed this issue when using the LSP with a slightly older version of neovim. I would occasionally have stale diagnostics that stuck around until I edited the line.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, examples/)
• I have updated the config schema (config-schema.json)
• I have added/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.

[calebdw]

This is not the correct solution, neovim had a bug in its implementation and that doesn't mean that we need to support such a case