Export Get-VsToolchainInfo so installer and patch builds run

[thejambi]

Quick Summary

build.ps1 imports only FwBuildHelpers.psm1, which imports FwBuildEnvironment.psm1 privately and re-exports a whitelist of its functions. Get-VsToolchainInfo was missing from that whitelist, so the toolset warning added in #1067 failed with "The term 'Get-VsToolchainInfo' is not recognized". CI never caught it because the only call site sits behind the -BuildInstaller/-BuildPatch guard, which CI.yml does not exercise.

Also export Get-CvtresDiagnostics from FwBuildEnvironment.psm1. FwBuildHelpers already lists it for re-export, but the inner module never exported it, so the name silently resolved to nothing and the next caller from build.ps1 would have hit the same failure.

CI-ready checklist

• Commit messages follow .github/commit-guidelines.md (subject ≤ 72 chars, no trailing punctuation; if body present, blank line then ≤ 80-char lines).
• No whitespace warnings locally:

  git fetch origin
  git log --check --pretty=format:"---% h% s" origin/<base>..
  git diff --check --cached

• Builds/tests pass locally (or I've run the CI-style build via build.ps1/test.ps1 or MSBuild).
• If this is core-developer AI-assisted work, I followed Docs/workflows/ai-pr-workflow.md and ran pr-preflight or the equivalent branch-readiness review before requesting review.
• For any Src/** folders touched, corresponding AGENTS.md files are updated or explicitly confirmed still accurate.

Notes for reviewers (optional)

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This change is 

[github-actions]

NUnit Tests

    1 files      1 suites   9m 5s ⏱️
5 778 tests 5 697 ✅ 81 💤 0 ❌
5 787 runs  5 706 ✅ 81 💤 0 ❌

Results for commit 5a3e1ab.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.07%. Comparing base (99858db) to head (5a3e1ab).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1072      +/-   ##
==========================================
+ Coverage   38.04%   38.07%   +0.02%     
==========================================
  Files        1499     1499              
  Lines      350127   350146      +19     
  Branches    40239    40239              
==========================================
+ Hits       133215   133307      +92     
+ Misses     187625   187560      -65     
+ Partials    29287    29279       -8     

see 124 files with indirect coverage changes

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