Fix discarded fire-and-forget Publish calls that ran inline and blocked their caller

[clrudolphi]

Summary

• Discarding a Task reference (_ = _mediator.Publish(...)) is not fire-and-forget: an async method's body runs synchronously up to its first genuine await, so a caller that never observes the returned Task can still be blocked for that entire synchronous prefix, and any exception the task throws goes unobserved.
• Confirmed real impact (traced during the LSP server: workspace/inlayHint/refresh and workspace/semanticTokens/refresh scale badly on large solutions, blocking unrelated requests for tens of seconds #471 investigation): a single .cs edit could make textDocument/didChange itself take several-to-tens of seconds to "complete" from the protocol's perspective — the entire reparse-every-open-feature-file cascade ran inline on the calling thread before the handler returned — blocking every other request queued behind it.
• Adds FireAndForgetExtensions (Reqnroll.IdeSupport.Common) with a Func<Task> overload that genuinely backgrounds work via Task.Run and a Task overload for work that's already independently running, both logging any fault instead of leaving it unobserved (an IIdeSupportLogger overload and a Microsoft.Extensions.Logging ILogger overload, for the two logging abstractions in use across the codebase).
• Replaces the four _ = _mediator.Publish(...) sites (BindingRegistryProviderRouter, MembershipIndex x2, LspWorkspaceScopeManager) and the LspServerConnectionService shutdown Task.Run, which lacked an outer catch around one non-critical-path await.
• Leaves the three genuinely-safe Task.Run sites that already carry their own full try/catch (ConnectorBindingRegistryProvider.RunDiscoveryAsync, CommentToggleCommandFilter) as-is — they never actually leave a fault unobserved, so wrapping them would add nothing.
• Also fixes a real bug surfaced by the change: the deferred Publish calls must use CancellationToken.None rather than the notification's own request-scoped token, since a background continuation that runs after the request completes would otherwise find that token already cancelled and silently drop the notification. Caught by the existing ProjectFilesDelta.feature spec, which failed until this was corrected.

Fixes #477

Test plan

• Added FireAndForgetExtensionsTests covering: the Func<Task> overload genuinely runs off the calling thread, a faulting Task/Func<Task> logs its exception instead of losing it, and a successful task logs nothing.
• Updated MembershipIndexTests / LspWorkspaceScopeManagerMembershipTests to poll for the now-genuinely-asynchronous Publish call instead of asserting immediately.
• dotnet test across Reqnroll.IdeSupport.Common.Tests, Reqnroll.IdeSupport.LSP.Server.Tests, and Reqnroll.IdeSupport.LSP.Server.Specs — all green, ProjectFilesDelta.feature's bound→unbound scenario in particular confirmed passing (it caught the CancellationToken bug above during development).
• Full solution build + full solution dotnet test — all green.

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