Fix documentation link clicks in the Info Panel

[NatElkins]

Summary

• register fsharp.showDocumentation in a form that matches how VS Code invokes hover command links
• pass the full DocumentationForSymbolRequest through the Info Panel path
• keep a fallback priming path for links that do not carry source context

Why

The Info Panel click path was not handling hover command arguments reliably, and older payloads depended on whatever cursor position happened to be active when the command ran. That is why the behavior could look timing-dependent.

With the FSAC change, the link payload can carry source context directly. This change consumes that payload in the extension and keeps the older fallback path when source context is missing.

Testing

• ./build.sh -t Build
• manual repro in fresh Extension Development Host windows using the patched FSAC build
• repeated checks of hover x then click Async, confirming the Info Panel opens Async docs instead of a blank panel

Notes

• depends on Resolve cref links in hover and documentation FsAutoComplete#1527
• refs Use cref etc. information from XML documentation to generate links in tooltips and Info Panel #1543