Perf: `.Result` blocks `MailboxProcessor` thread in `FSharpProjectOptionsManager`

## Description

`FSharpProjectOptionsManager.fs`'s `hasDependentVersionChanged` used blocking `.Result` calls on `GetDependentVersionAsync(ct).Result` when comparing non-F# project references. Since this function is invoked from `isProjectInvalidated`, which is called from `tryComputeOptions` / `tryComputeOptionsBySingleScriptOrFile` inside the `FSharpProjectOptionsReactor`'s `MailboxProcessor` loop, these synchronous waits could block the single-threaded reactor loop during project invalidation, causing UI/background responsiveness issues.

## Repro / Evidence

Identified via CPU profiling of background F# editor activity in Visual Studio; located via code search for `.Result` usages in `vsintegration/src/FSharp.Editor`.

## Fix

Convert `hasDependentVersionChanged` and `isProjectInvalidated` to `cancellableTask` workflows using `let!` instead of `.Result`, so the invalidation path is fully async and no longer blocks the mailbox loop.

## Priority

P2 (medium-term)

Start in vsintegration/src/FSharp.Editor/FSharpProjectOptionsManager.fs, focusing on hasDependentVersionChanged and isProjectInvalidated, then trace their calls from tryComputeOptions and tryComputeOptionsBySingleScriptOrFile in the FSharpProjectOptionsReactor MailboxProcessor loop. Replace the blocking dependent-version waits with cancellableTask workflows and verify that the invalidation path remains fully asynchronous without blocking the mailbox loop.

Written by the indexing model from the issue text.