Fix external inputs breaking base parameter spreads

[polatengin]

Description

Fixes #19954.

When a parent parameter file contains an externalInput, using the spread operator with another inherited parameter could fail with BCP338.

In this PR;

I've added tests that reproduce the issue with ...base.B

I've fixed the issue in ExpressionBuilder by evaluating only the selected base parameter

I've also added coverage for object and array spreads, property access, bracket access, and nested parameter files

Checklist

• I have read and adhere to the contribution guide.

Microsoft Reviewers: Open in CodeFlow

[github-actions]

Test this change out locally with the following install scripts (Action run 29442789778)

VSCode

• Mac/Linux

  bash <(curl -Ls https://aka.ms/bicep/nightly-vsix.sh) --run-id 29442789778

• Windows

  iex "& { $(irm https://aka.ms/bicep/nightly-vsix.ps1) } -RunId 29442789778"

Azure CLI

• Mac/Linux

  bash <(curl -Ls https://aka.ms/bicep/nightly-cli.sh) --run-id 29442789778

• Windows

  iex "& { $(irm https://aka.ms/bicep/nightly-cli.ps1) } -RunId 29442789778"