improving idempotency of registration methods for AspNet and App Insights

[brettsam]

Fixes #1301

Both the AspNet and App Insights packages had some issues with calling Configure...() multiple times. This prevents the issues by checking and bailing early if a method has already been called to register services.

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

[Copilot]

Pull request overview

This PR improves the idempotency of registration methods for both AspNet Core integration and Application Insights packages, addressing issue #1301. The changes ensure that calling configuration methods multiple times doesn't result in duplicate service registrations or errors.

Key Changes:

• Implemented idempotency checks in configuration methods using different strategies: IHostBuilder.Properties sentinel values and IServiceCollection.Any() checks
• Changed WorkerOptions.Capabilities from using Add() to indexer syntax to prevent duplicate key exceptions
• Added comprehensive tests to verify idempotent behavior while ensuring user callbacks are still invoked each time

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
extensions/Worker.Extensions.Http.AspNetCore/src/FunctionsHostBuilderExtensions.cs	Added idempotency check using IHostBuilder.Properties with sentinel key to prevent duplicate service registration
extensions/Worker.Extensions.Http.AspNetCore/src/WorkerBuilderExtensions.cs	Added idempotency check using ServiceCollection.Any() and changed Capabilities.Add() to indexer syntax
src/DotNetWorker.ApplicationInsights/FunctionsApplicationInsightsExtensions.cs	Added idempotency check using ServiceCollection.Any() to detect marker service
test/extensions/Worker.Extensions.Http.AspNetCore.Tests/FunctionsHostBuilderExtensionsTests.cs	Added test verifying idempotent behavior with multiple calls while ensuring callbacks execute each time
test/DotNetWorker.Tests/ApplicationInsights/ApplicationInsightsConfigurationTests.cs	Added count assertions to verify services are only registered once despite multiple configuration calls

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