[René Larch (r-Larch)]

Fixes #508. Also fixes #481 (same change as #482, in a separate commit that can be dropped if #482 lands first).

• PooledBuffer.FromStringUtf8 / FromSpanUtf8: write the NUL terminator explicitly. On .NET the buffer comes from ArrayPool<byte>.Shared, which is not zeroed, so napi_throw_*_error, node_embedding_runtime_config_add_module and node_embedding_last_error_message_set read stale bytes past the string.
• An empty string is encoded as just the terminator instead of PooledBuffer.Empty (null pointer), so JSError.ThrowError("") now actually throws. null still maps to a null pointer (used for the optional code argument).
• EmbeddingRuntimeConfigAddModule: dispose the module-name buffer. Node.js copies the name into a std::string (EmbeddedRuntime::AddModule), so this is safe.
• Utf8StringArray: write each terminator explicitly (Embedding args occasionally get trailing garbage on .NET (missing null terminator on a pooled buffer) #481). This is needed here because the new tests dirty the shared pool before the Node.js platform is created.

Tests

All fail on main and pass with the fix:

• Utf8BufferTests: dirties the shared pool, then checks that both PooledBuffer helpers and Utf8StringArray produce NUL-terminated UTF-8 for "", ASCII and non-ASCII strings, and that null still pins to a null pointer.
• NodejsEmbeddingTests.ThrowErrorFromCallback("x" / ""): throws from a .NET callback with a dirty pool and checks the JS e.message. Before: x followed by 15 garbage bytes, and "" did not throw at all.
• New helper TestUtils.FillArrayPoolWithGarbage() (no-op on .NET Framework).

Local run (Windows x64, .NET SDK 10.0.402): net10.0 157/159, net9.0 126/127, net8.0 126/127. The only failure, napi-dotnet/overloads, fails the same way on main. I could not run net472 locally (the test-case builds fail with MSB4062 on this machine, also on main).

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🟢 Approval recommended

The focused fixes have regression coverage, and static review identified no blocking issues.

No open issues 🎉

What changed in this PR

Fixes pooled UTF-8 string termination for native Node.js calls, addressing #508 and #481.

Changes:

• Explicitly writes NUL terminators and distinguishes empty strings from null.
• Disposes the module-name buffer after registration.
• Adds regression tests using dirty pooled buffers.

File	Description
test/​Utf8BufferTests.cs	Tests UTF-8 termination and null handling.
test/​TestUtils.cs	Adds a helper to dirty pooled buffers.
test/​NodejsEmbeddingTests.cs	Tests callback error messages, including empty strings.
src/​NodeApi/​Runtime/​Utf8StringArray.cs	Explicitly terminates each encoded string.
src/​NodeApi/​Runtime/​PooledBuffer.cs	Terminates UTF-8 buffers and preserves null semantics.
src/​NodeApi/​Runtime/​NodejsRuntime.Embedding.cs	Disposes the module-name buffer.

_{Review effort: Balanced}

[René Larch (r-Larch)]

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