Fix ScriptTagHelper regression with importmap scripts containing explicit content

[Copilot]

The ScriptTagHelper was incorrectly suppressing <script type="importmap"> tags that contained explicit user-provided content, causing them to render as nothing in ASP.NET Core 9.0.

Problem

When users wrote importmap scripts with explicit content like:

<script type="importmap">
{
  "imports": {
    "jquery": "https://code.jquery.com/jquery.js"
  }
}
</script>

The ScriptTagHelper would completely suppress the output instead of preserving the user's content. This regression broke existing applications that relied on explicit importmap definitions.

Root Cause

The tag helper was being overly aggressive - it processed all script tags with type="importmap" and when no ImportMapDefinition was found in endpoint metadata, it would call output.SuppressOutput() regardless of whether the user intended to use the automatic importmap feature.

Solution

Modified the ScriptTagHelper to only suppress output when there's clear intent to use the automatic importmap functionality:

• If asp-importmap attribute is present but no definition is found → suppress output (existing behavior preserved)
• If ImportMap property is explicitly set but null → suppress output (existing behavior preserved)
• If neither condition is met → let the tag render normally with user content

This allows users to continue using explicit importmap content while preserving all existing functionality for automatic importmap generation.

Testing

Added comprehensive tests to verify:

• Explicit importmap content is preserved when no asp-importmap attribute is used
• Output is still suppressed when asp-importmap is used but no definition exists
• All existing functionality remains intact (51/51 existing tests pass)

Fixes #58973.

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