Duplication appsettings.{Environment}.json in configuration sources

### Description

### 🔍 Description

in `FunctionsApplication.CreateBuilder(args)`, `HostApplicationBuilder` constructor already applies default host/app configuration and default services in its constructor (when defaults are enabled).
In the current startup flow, `ConfigureDefaults()` is also being invoked, which appears redundant and will cause duplicate/default configuration sources (i.e. appsettings.json and appsettings.{Environment}.json).

This creates confusion about expected hosting setup and could lead to subtle ordering differences compared to intended `HostApplicationBuilder` usage.

### 💡 Expected behavior

When using `HostApplicationBuilder` with defaults enabled, the worker initialization path should not re-apply `IHostBuilder.ConfigureDefaults(...)`.

There should be a single, clear defaulting path to avoid duplicate configuration/service wiring.

### 📌 Actual behavior

Current flow appears to:
1. Construct `HostApplicationBuilder` (which applies defaults), and
2. Also call `ConfigureDefaults(...)` through the host-builder path.

### ✅ Proposed change

- Audit startup/hosting construction path(s) that use `HostApplicationBuilder`.
- Remove `ConfigureDefaults(...)` invocation on _bootstrapHostBuilder.

### 🧪 Notes

From `dotnet/runtime` behavior:
- `HostApplicationBuilder` constructor applies default config/services directly.
- Calling `ConfigureDefaults()` afterward is generally redundant and will introduce duplicate setup.

### 📚 Environment

- Version: Microsoft.Azure.Functions.Worker 2.51.0
- Repo: `Azure/azure-functions-dotnet-worker`
- Related upstream behavior: `dotnet/runtime` `HostApplicationBuilder`
- Date observed: 2026-03-02

### Steps to reproduce

1. clone this minimal reproduction repo: https://github.com/sinapy/SampleAzureWorkerDuplicateConfiguration
2. Set a breakpoint after the `var configurationSources = builder.Configuration.Sources;` line in Program.cs
3. See how the configuration sources for appsettings.Development.json and appsettings.json is duplicated

Start at the FunctionsApplication.CreateBuilder(args) startup path and inspect how HostApplicationBuilder and _bootstrapHostBuilder are constructed and configured. Use the linked minimal reproduction and the breakpoint in Program.cs to verify the configuration sources; done means appsettings.json and appsettings.{Environment}.json are each registered once and the redundant defaulting path is absent.

Written by the indexing model from the issue text.