DI Refactor: Azure.Mcp.Tools.Pricing

[Copilot]

PricingGetCommand was resolving IPricingService at call-time via context.GetService<T>() rather than through constructor injection, inconsistent with the project-wide DI refactor (#158).

Changes

• PricingGetCommand: Added IPricingService constructor parameter; replaced context.GetService<IPricingService>() with injected _pricingService field

// Before
public sealed class PricingGetCommand(ILogger<PricingGetCommand> logger) : ...
{
    // In ExecuteAsync:
    var pricingService = context.GetService<IPricingService>();
    var prices = await pricingService.GetPricesAsync(...);
}

// After
public sealed class PricingGetCommand(ILogger<PricingGetCommand> logger, IPricingService pricingService) : ...
{
    private readonly IPricingService _pricingService = pricingService;
    // In ExecuteAsync:
    var prices = await _pricingService.GetPricesAsync(...);
}

• PricingGetCommandTests: Updated all PricingGetCommand constructor calls to pass _pricingService directly; PricingSetup.cs required no changes as IPricingService and PricingGetCommand were already registered as singletons.

Original prompt

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This section details on the original issue you should resolve

<issue_title>DI Refactor: Azure.Mcp.Tools.Pricing</issue_title>
<issue_description>## Summary

Refactor Azure.Mcp.Tools.Pricing to use constructor dependency injection instead of resolving services via context.GetService<T>() in ExecuteAsync.

This is a sub-issue of #158.

Instructions

Follow the algorithm described in tools/di-refactor-plan.md with {ToolArea} = Pricing.

An example PR is: #1815
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes DI Refactor: Azure.Mcp.Tools.Pricing #1866

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