Investigate enabling proxy support for Fabric MCP

Nobody has claimed this yet.

## Description
During the migration from `HttpClientService` to `IHttpClientFactory` (PR https://github.com/microsoft/mcp/pull/1564), it was observed that Fabric MCP does not have proxy configuration enabled.

## Historical Behavior
Fabric's Program.cs originally called `AddAzureTenantService(addUserAgentClient: false)`. Despite param name, this flag disabled all HTTP client defaults including proxy support from environment variables .

## Current Behavior

In the linked pr, the historical behavior was preserved. Fabric now calls `AddHttpClientServices()` without `configureDefaults: true`, maintaining the same proxy-disabled behavior but with a more explicit parameter name.

#### Note: Other defaults (timeout, UserAgent) are not a major concern since:

* Timeout: Our 100s default matches .NET's default
* UserAgent: Fabric explicitly sets "OneLake MCP" per-request

## Question

Should Fabric enable proxy support for users in corporate environments behind proxies?

Note: This would be a change to how Fabric consumes the `Azure.Mcp.Core` API, and potential api (param likely) change to `Azure.Mcp.Core:: AddHttpClientServices `

Start with Fabric's Program.cs and the AddHttpClientServices API in Azure.Mcp.Core, then review the migration described in PR 1564. Determine whether proxy support should be enabled and whether the configureDefaults parameter needs an API change; done means the decision and resulting behavior are documented and covered by the relevant tests.

Written by the indexing model from the issue text.