Write Minimal API views directly to the response PipeWriter

[sebastienros]

Minimal API view responses currently pass through both TextWriterFluidOutput and StreamWriter, adding a redundant character buffer and per-response UTF-8 conversion allocations. This change writes Fluid output directly to HttpResponse.BodyWriter while retaining bounded buffering and streaming behavior.

Changes

• Add a pooled UTF-8 PipeWriterFluidOutput implementation of IFluidOutput.
• Preserve encoder state across writes, including surrogate pairs split at buffer boundaries.
• Flush at OutputBufferSize, honor request cancellation, and leave the ASP.NET response pipe open.
• Use the new output directly from ActionViewResult.
• Cover UTF-8 output, buffer growth, automatic flushing, cancellation, and disposal behavior.

Performance

A 20,000-iteration rendering-shaped microbenchmark improved from 27.7 ms and 5,384 B/operation to 18.1 ms and 120 B/operation.

Validation

• Release build succeeds for net8.0, net9.0, and net10.0.
• Focused pipe checks cover UTF-8, split surrogate pairs, bounded flushing after buffer growth, and cancellation.
• The full xUnit suite could not be restored locally because the configured package proxy does not yet contain xunit.v3.mtp-v2 4.0.0.