[browser][coreCLR] idempotent lazy download of dll/pdb

[pavelsavara]

Summary

Loading the same lazy assembly more than once (e.g. when Blazor fires OnNavigate
repeatedly) incorrectly threw:

<assembly> must be marked with 'BlazorWebAssemblyLazyLoad' item group in your project file to allow lazy-loading.

even though the assembly was correctly marked and had already been loaded once.
This change makes a repeated loadLazyAssembly call an idempotent no-op that
returns false, and adds a regression test.

Root cause

The lazy loader matched and de-duplicated assets using the asset's virtualPath
field. However, fetchAssembly calls normalizeVirtualPath, which mutates
asset.virtualPath in place — it rewrites the .wasm extension to .dll and
prefixes the path with the app base (and culture). The lazyAssembly resources
array holds references to those same asset objects.

As a result, after the first successful load:

1. The asset's virtualPath no longer equals the bare Json.dll / Json.wasm
   name the lookup compared against, so the second call failed to find the asset.
2. The dedup set (loadedLazyAssemblies) stored the already-rewritten path, so
   the early-return guard never matched on the second call either.

With the asset "not found," the loader fell through to the
must be marked with 'BlazorWebAssemblyLazyLoad' error path.

Changes

src/native/libs/Common/JavaScript/loader/assets.ts

• Added a small lazyAssetFileName(virtualPath) helper that extracts just the
  file name (the segment after the last /), so asset matching is robust to the
  in-place normalization that prefixes the path.
• Matching of the DLL/WASM asset and the PDB asset now compares the file name
  rather than the full virtualPath.
• De-duplication now keys off the stable assemblyNameWithoutExtension instead of
  the mutable virtualPath, and the dedup check is moved before the asset
  lookup so a repeated load short-circuits to return false immediately.

src/mono/wasm/testassets/WasmBasicTestApp/App/wwwroot/main.js

• Captures and reports the boolean result of loadLazyAssembly (firstJsonLoad).
• When the loadLazyAssemblyTwice=true query string is set, loads the same lazy
  assembly a second time and reports secondJsonLoad, exercising the idempotent
  path.

src/mono/wasm/Wasm.Build.Tests/LazyLoadingTests.cs

• Added LoadLazyAssemblyTwiceIsIdempotent, a regression test that:
  • asserts the first load returns true,
  • asserts the second load of the same assembly returns false (idempotent no-op),
  • asserts the test still produces its expected JSON output, and
  • asserts the must be marked with 'BlazorWebAssemblyLazyLoad' error is not
    emitted to the console.

Testing

Added LoadLazyAssemblyTwiceIsIdempotent to Wasm.Build.Tests, which reproduces
the failure without the fix and passes with it.

[Copilot]

Pull request overview

This PR makes browser lazy-assembly loading idempotent by making the loader’s “already loaded” tracking and asset lookup resilient to virtualPath being rewritten during the first fetch (e.g., normalization that prefixes _framework/).

Changes:

• Update lazy assembly/PDB lookup to match by asset filename (basename) instead of full virtualPath, and track loaded lazy assemblies by the assembly’s basename (without extension).
• Add a regression test that loads the same lazy assembly twice and verifies the second call is a no-op (returns false) and does not throw.
• Extend the WASM test asset’s main.js to exercise and log the two consecutive lazy-load calls.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/native/libs/Common/JavaScript/loader/assets.ts	Makes lazy assembly/PDB matching and deduplication robust against in-place virtualPath normalization/mutation.
src/mono/wasm/Wasm.Build.Tests/LazyLoadingTests.cs	Adds regression coverage verifying repeated lazy-load calls are idempotent and don’t surface the “must be marked” error.
src/mono/wasm/testassets/WasmBasicTestApp/App/wwwroot/main.js	Adds a test hook to attempt a second lazy load and emit observable results for the new test.