[Blazor] Fix JSInitializer URL computation for base URLs with query parameters

[Copilot]

The JSInitializer adjustPath function was using string concatenation to build URLs, which created malformed URLs when the document base URI contained query parameters or other URL components.

Problem:
When document.baseURI is http://domain?a=x, the string concatenation approach would create:

http://domain?a=x/_content/PackageId/file.js

This is a malformed URL with the path incorrectly placed after the query string.

Solution:
Replace string concatenation with the URL constructor, following the same pattern used in MonoPlatform.ts:

// Before (problematic)
path = base.endsWith('/') ? `${base}${path}` : `${base}/${path}`;

// After (fixed)
return new URL(path, document.baseURI).toString();

Result:
The same scenario now correctly produces:

http://domain/_content/PackageId/file.js

This change ensures proper URL resolution according to web standards, where query parameters and fragments in the base URL are handled correctly when resolving relative paths.

Added comprehensive tests covering various URL scenarios including query parameters, fragments, trailing slashes, and subdirectories.

Fixes #63184.

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[Copilot]

Pull Request Overview

This PR fixes a URL resolution bug in the JSInitializer's adjustPath function where string concatenation was creating malformed URLs when the document base URI contained query parameters or other URL components.

Key changes:

• Replaced string concatenation with the URL constructor for proper URL resolution
• Aligned implementation with the pattern used in MonoPlatform.ts
• Added comprehensive test coverage for various URL scenarios