Remove AI assistant references from VS Code extension

After removing the AI assistant feature from the dashboard (#18726), the VS Code extension still has leftover references that should be cleaned up:

1. **`extension/src/debugger/languages/dotnet.ts`** — Injects `ASPIRE_DASHBOARD_AI_DISABLED=true` env var for every AppHost launch. The dashboard no longer recognizes this variable.

2. **`extension/src/debugger/languages/maui.ts`** — Lists `ASPIRE_DASHBOARD_AI_DISABLED` in `isKnownMauiApplicationEnvironmentVariable`.

3. **`extension/src/dcp/DashboardTelemetryPassthrough.ts`** — Allowlists 5 `Aspire.Dashboard.AIAssistant.*` property keys in `ALLOWED_DASHBOARD_PROPERTY_KEYS` and `ALLOWED_STRING_ARRAY_PROPERTY_KEYS`.

4. **`extension/src/test/dashboardTelemetryPassthrough.test.ts`** — Tests exercise the removed AI property keys.

These are harmless (they emit a now-ignored env var and allowlist telemetry keys that will never arrive), but should be removed to keep the extension in sync with the dashboard.

Start with the four paths named in the issue: the .NET and MAUI debugger files, extension/src/dcp/DashboardTelemetryPassthrough.ts, and extension/src/test/dashboardTelemetryPassthrough.test.ts. Review the AI-related environment-variable and telemetry-key references, then run the dashboard telemetry passthrough tests. Done means the obsolete references and their tests are removed without breaking the remaining extension tests.

Written by the indexing model from the issue text.