fix: close structural telemetry gaps (issue #255)

[clrudolphi]

Part of #255

Summary

• Rider had no consumer for telemetry/event at all, so every already-implemented event (discovery, go-to-step, rename, comment-toggle, find-unused-steps, go-to-hook) produced zero telemetry data for Rider users. Adds ReqnrollTelemetryEventInterceptor + RiderTelemetryTransmitter, giving Rider parity with VS's TelemetryEventInterceptor and VS Code's telemetry.ts by forwarding to the same Application Insights resource. The telemetryEvent hook point was confirmed against JetBrains' public mirror at the exact pinned platform version (Rider 2024.3.5 / branch 243), not guessed.
• The LSP server was wired with NullTelemetryService for ITelemetryService, so MonitorError calls from LSP.Core parsing (DeveroomGherkinParser/DeveroomTagParser) were silently dropped in production. Adds LspErrorTelemetryService, which forwards MonitorError to ILspTelemetryService as an Error event (redacting filesystem paths from the message per the architecture doc's privacy requirement) while keeping every other host-UI-only member a no-op.

Update: also folds in an Interface Segregation refactor of ITelemetryService, prompted by review discussion on this PR. Only 1 of its ~16 members (MonitorError) was ever genuinely shared between LSP.Core and the VS host — every other member is a VS-lifecycle concern (project wizards, welcome/upgrade dialogs) that forced every LSP-side implementation to stub out ~15 unrelated no-ops. This commit:

• Adds IErrorTelemetryService (MonitorError only); ITelemetryService now extends it. DeveroomGherkinParser, DeveroomTagParser, and CompletionContextResolver depend on the narrow interface directly. IIdeScope.TelemetryService stays typed as the full ITelemetryService (VS's wizard flows genuinely need it) — both interfaces resolve to the same DI singleton on the LSP server.
• Fixes a second, previously-unnoticed silent-drop bug this surfaced: LspIdeScope.TelemetryService was hardcoded to NullTelemetryService instead of the DI-registered service, so errors reported through that path (e.g. WatchedFilesHandler's config-load exceptions) were still being dropped even after the MonitorError fix above.
• Closes 4 legacy-compatibility gaps found while auditing the interface: wires up MonitorOpenFeatureFile (implementation was real, never called — hooked into VsProjectEventMonitor's document-activation phase machine) and MonitorExtensionDaysOfUsage (WelcomeService tracked the count but never reported it); restores the "Extension loaded" signal (MonitorOpenProjectSystem) at its new trigger point; deletes MonitorLoadProjectSystem (confirmed fully inert everywhere). MonitorParserParse/MonitorReqnrollGeneration are documented rather than wired — the former is superseded by perf-sampling telemetry, the latter has no live feature to hook into.

Structural gaps only — usage-event additions are #260, also part of #255. Not using a closing keyword here since #255 isn't fully addressed until both PRs land.

Test plan

• dotnet build Reqnroll.IdeSupport.slnx — clean, 0 warnings/errors
• dotnet test tests/LSP/Reqnroll.IdeSupport.LSP.Server.Tests — 622/622 passing
• dotnet test tests/VisualStudio/Reqnroll.VisualStudio.Tests — 118/118 passing
• dotnet test tests/VisualStudio/Reqnroll.IdeSupport.VisualStudio.Wizards.Tests — 65/65 passing (2 new tests for MonitorExtensionDaysOfUsage)
• dotnet test tests/LSP/Reqnroll.IdeSupport.LSP.Server.Specs — 144/144 passing (18 pre-existing skips, unrelated)
• Rider side (ReqnrollTelemetryEventInterceptor/RiderTelemetryTransmitter) could not be built/tested in this environment — the sandbox's JVM can't complete the Gradle wrapper's TLS handshake to download Gradle 8.10 (unrelated to the code change; reviewed manually against the real platform API, but needs a ./gradlew build/runIde pass before merge)
• MonitorOpenFeatureFile's new wiring in VsProjectEventMonitor (DTE/COM-heavy) has no automated coverage — the existing test file for this class only covers the IVsTrackProjectDocumentsEvents2 file-tracking half, not the WindowActivated flow; the addition is defensively try/caught so it can't break document activation, but hasn't been verified live in VS

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