Remove package source mapping to enable VMR backflow

[Copilot]

Package source mapping in NuGet.config is incompatible with VMR backflow operations - stable backflow requires NuGet.config updates on every flow, and internal validation backflow requires extensive mapping rework per run. Limited security value given all feeds are MS-owned.

Changes

• NuGet.config: Removed <packageSourceMapping> section (all 12 package sources remain intact)
• eng/Analyzer.props: Added NU1507 to NoWarn to suppress package source mapping warnings

<!-- Suppress NU1507 (Package source mapping is not compatible with VMR backflow operations) -->
<NoWarn>$(NoWarn);NU1507</NoWarn>

Original prompt

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This section details on the original issue you should resolve

<issue_title>VMR backflow is incompatible with package source mapping</issue_title>
<issue_description>winforms added package source mapping to its Nuget.config. This is incompatible with some practices:

• Stable backflow (requires updates to Nuget.config on every new flow)
• Internal validation backflow (e.g. https://dev.azure.com/dnceng/internal/_build/results?buildId=2884334&view=logs&j=fdea2036-e6b2-512d-a259-98f88c972586&t=f9d531e2-a847-5dff-8213-c40306ac90d0). Internal backflow adds internal feeds and requires a large rework of the package source mapping on every run.

I recommend removing the package source mapping and suppressing NU1507. The package source mapping is of limited utility given these are all MS owned feeds.</issue_description>

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

• Fixes VMR backflow is incompatible with package source mapping #14232

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.19200%. Comparing base (9c45a8c) to head (9e83b48).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #14233         +/-   ##
===================================================
+ Coverage   77.18007%   77.19200%   +0.01192%     
===================================================
  Files           3279        3279                 
  Lines         645129      645129                 
  Branches       47728       47728                 
===================================================
+ Hits          497911      497988         +77     
+ Misses        143538      143462         -76     
+ Partials        3680        3679          -1     

Flag	Coverage Δ
Debug	77.19200% <ø> (+0.01192%)	⬆️
integration	18.99716% <ø> (+0.01533%)	⬆️
production	52.08046% <ø> (+0.02676%)	⬆️
test	97.40479% <ø> (ø)
unit	49.53997% <ø> (+0.04692%)	⬆️

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