Remove ActivityDisposable conditional directives and replace with System.IDisposable | null

[Copilot]

This PR removes the conditional compilation directives for ActivityDisposable that were needed during the nullness bootstrapping phase when the "last known good" compiler had bugs around 'use' and nullable disposable handling.

Changes Made

1. Removed conditional compilation block from NullnessShims.fs:

   // REMOVED:
   #if BUILDING_WITH_LKG
       type ActivityDisposable = System.IDisposable
   #else
       type ActivityDisposable = System.IDisposable | null
   #endif

2. Updated function signatures in Activity.fs and Activity.fsi to directly use System.IDisposable | null:

   • start: name: string -> tags: (string * string) seq -> System.IDisposable | null
   • startNoTags: name: string -> System.IDisposable | null
   • startAndMeasureEnvironmentStats: name: string -> System.IDisposable | null

Impact

• Simplifies the codebase by removing conditional compilation complexity
• All ActivityDisposable references are now directly replaced with System.IDisposable | null
• No functional changes to behavior - the type alias was already resolving to System.IDisposable | null in the non-LKG case
• Maintains nullness safety while removing the bootstrapping workaround

The changes are minimal and surgical, affecting only the necessary files to complete the cleanup of the nullness bootstrapping infrastructure.

Fixes #18706.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• 1k9vsblobprodcus379.vsblob.vsassets.io
  • Triggering command: dotnet restore FSharp.Compiler.Service.sln (dns block)
• 4vyvsblobprodcus361.vsblob.vsassets.io
  • Triggering command: dotnet restore FSharp.Compiler.Service.sln (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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[github-actions]

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cc @dotnet/fsharp-team-msft