Fix ContentTypeProvider to respect custom content-type mappings precedence

[Copilot]

This PR fixes an issue where custom content-type mappings were being ignored in favor of built-in mappings in the Static Web Assets ContentTypeProvider.

Problem

In .NET SDK versions 9.0.300+, the ContentTypeProvider was prioritizing built-in mappings over custom mappings when both existed for the same file pattern. This prevented users from overriding default cache behavior for common file types like *.html and *.js.

The issue occurred in the TryGetMapping method where built-in mappings were checked first:

if (_builtInMappings.TryGetValue(match.Pattern, out mapping) || _customMappings.TryGetValue(match.Pattern, out mapping))

Since built-in mappings for *.html and *.js have Cache = null, they would always win over custom mappings that might specify cache headers like "no-cache" or "max-age=3600".

Solution

The fix swaps the lookup order to check custom mappings first:

if (_customMappings.TryGetValue(match.Pattern, out mapping) || _builtInMappings.TryGetValue(match.Pattern, out mapping))

This minimal one-line change ensures that:

• Custom mappings take precedence over built-in mappings when both exist
• User configuration properly overrides defaults as expected
• Built-in mappings still serve as fallbacks when no custom mapping exists
• No breaking changes to existing functionality

Testing

Added comprehensive regression tests to verify:

• Custom *.html mappings with cache settings override built-in mappings
• Custom *.js mappings with cache settings override built-in mappings
• Custom mappings work correctly for compressed files (.gz)
• All existing functionality continues to work as before

This restores the expected behavior from SDK versions 9.0.101–9.0.205 where user configuration properly overrode built-in defaults.

Fixes #50152.

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[Copilot]

Pull Request Overview

This PR fixes a precedence issue in the Static Web Assets ContentTypeProvider where built-in content-type mappings were overriding custom user-defined mappings. The fix ensures custom mappings take priority over built-in defaults, allowing users to properly override cache behavior for file types like HTML and JavaScript.

• Swapped lookup order in TryGetMapping to check custom mappings before built-in mappings
• Added comprehensive regression tests for HTML, JavaScript, and compressed file scenarios
• Restored expected behavior where user configuration overrides built-in defaults

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/StaticWebAssetsSdk/Tasks/Data/ContentTypeProvider.cs	Fixed mapping precedence by checking custom mappings first in TryGetMapping method
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/ContentTypeProviderTests.cs	Added three test cases to verify custom mappings override built-in mappings for HTML, JS, and compressed files