Update roslyn to 5.12.0-1.26471.3

[dibarbet]

Roslyn build: 20260921.3

View Complete Diff of Changes

• Handle empty diagnostic help links (PR: #85734)
• Fix Razor crash for incomplete render modes (PR: #85693)
• Avoid constructor-time ownership for compiler client mutexes (PR: #85675)
• Special-case nullable walker to understand lock (null) in legacy mode (PR: #85182)
• Cleanup Razor integration tests (PR: #85701)
• Remove OmniSharp external access projects (PR: #85700)
• Route faults through the current telemetry instance (PR: #85273)
• Defer LSP semantic tokens until projects finish loading (PR: #85688)
• Support public pull diagnostics alongside legacy endpoints (PR: #85208)
• Create per-server daemon telemetry sessions (PR: #85155)
• Remove analyze and refinement session support (PR: #85055)
• Unsafe evolution: fix new() constraint handling in declarations (PR: #84888)
• Update to .NET 11 RC1 (PR: #85228)
• Set LanguageServer and BuildHost handles to not be inheritable (PR: #85120)
• Merge Helix test history across retry runs (PR: #85679)
• Use a fixture instead of statically writing global.json to a high part of shared temp directory (PR: #85668)
• Restore the built-in VS SDK .pkgdef generation and migrate to it (PR: #85231)
• Improve IOperation shape for a cast utilizing user-defined conversions (PR: #85262)

[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved review issues were identified.

Review effort: Lite
Findings: None

What changed in this PR

Updates the bundled Roslyn language server to 5.12.0-1.26471.3 and documents related user-facing improvements.

Changes:

• Bumps the Roslyn dependency version.
• Adds selected Roslyn improvements to the changelog.

File	Description
package.json	Updates the Roslyn dependency version.
CHANGELOG.md	Documents relevant user-facing Roslyn changes.

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