Drafted PR -- PSES PrepareRename/Rename null RenameCapability guard (#2297)

Status: PR-READY, NOT SUBMITTED. The fix branch is pushed to Mike's fork:

• Fork branch: manderse21/PowerShellEditorServices @ fix/2297-prepare-rename-null-capability
• Base: PowerShell/PowerShellEditorServices @ main (which is v4.6.0, commit d2112c21)
• Commit: Fix #2297: guard null RenameCapability in rename handlers

Opening the PR (fork branch -> upstream main) is Mike's explicit action. Nothing
has been submitted, commented, or posted upstream.

PR title

Fix #2297: guard null RenameCapability in rename handlers

PR body (draft)

Summary

PrepareRenameHandler and RenameHandler both read capability.PrepareSupport in
GetRegistrationOptions. The language-server framework passes a null
RenameCapability when the client's initialize omits textDocument.rename, so the
dereference throws a NullReferenceException during capability registration. The
exception leaves the initialize request unanswered, so the handshake hangs for
any client that legitimately omits the optional rename capability (#2297).

The rename provider is new in v4.6.0 (#2292), so this affects v4.6.0.

Fix

Guard both handlers with a property pattern, so an absent capability is treated as
"no prepare support" instead of dereferenced:

// before
public RenameRegistrationOptions GetRegistrationOptions(RenameCapability capability, ClientCapabilities clientCapabilities)
    => capability.PrepareSupport ? new() { PrepareProvider = true } : new();

// after
public RenameRegistrationOptions GetRegistrationOptions(RenameCapability capability, ClientCapabilities clientCapabilities)
    => capability is { PrepareSupport: true } ? new() { PrepareProvider = true } : new();

Tests

Adds a regression test to each handler's test class asserting
GetRegistrationOptions(null, ...) does not throw and reports no prepare provider.

Validation (performed locally before submission)

• dotnet build -c Release is clean (the Release configuration's documentation/analyzer
  gate passes).
• The rename test category is green: 102 tests including the two new regression tests
  (Category=PrepareRename|Category=RenameHandlerFunction, net8.0 Release).
• Adversarial control: reverting only the guard makes both new tests fail with the
  exact NRE -- RenameHandler.GetRegistrationOptions (RenameHandler.cs:37) and
  PrepareRenameHandler.GetRegistrationOptions (RenameHandler.cs:23) -- and they pass
  again with the guard restored, confirming the tests guard the regression and the fix
  is the cause.
• End-to-end: an initialize that omits rename, sent over -Stdio, never returns
  an initialize response against a stock v4.6.0 bundle (hang reproduced); the same
  probe against a bundle in which only Microsoft.PowerShell.EditorServices.dll is
  rebuilt from this branch returns the initialize response and completes the handshake.
• The full PowerShellEditorServices.Test unit project runs 315/320 green on net8.0
  Release. The 5 failures are pre-existing environment dependencies unrelated to this
  change -- CanLoadPSReadLine, CanLoadPSScriptAnalyzerAsync, and three PSSA-dependent
  tests (parse-error / script-marker / built-in command help) -- which need PSReadLine and
  PSScriptAnalyzer provisioned (this local sandbox does not install them; the project's CI
  does). No rename test fails.

Closes #2297.