Feature request: opt-in setting to clear diagnostics immediately on didChange

**Is your feature request related to a problem? Please describe.**

When pyright is used as a language server by AI coding tools, stale diagnostics cause problems. The tool edits a file via `textDocument/didChange`, then reads diagnostics to decide what to do next. Because reanalysis is async, the diagnostics it reads are from the previous state. This causes the tool to re-fix already-fixed errors, miss newly introduced errors, or loop on diagnostics that no longer apply.

This isn't a problem for interactive editors where a human is still typing and the debounce is desirable.

**Describe the solution you'd like**

An opt-in setting (`pyright.eagerDiagnosticInvalidation`, default `false`) that publishes empty diagnostics for a file immediately on `didChange`, before reanalysis completes. No behavior change for anyone who doesn't enable it.

The setting would be configured by tool vendors in their LSP initialization config, not by end users.

I have a working implementation with tests if there's interest.

Start by locating the language-server handling for textDocument/didChange and the initialization settings for Pyright. Verify how diagnostics are published before and after asynchronous reanalysis, then add coverage for the opt-in setting, ensuring enabled changes publish empty diagnostics immediately while the default remains unchanged.

Written by the indexing model from the issue text.