Size template buffers from archive entries

[JeremyKuhne]

Summary

Follow-up to #56185 and #56163.

• expose archive-entry length through an internal IKnownLengthFile
• size processing buffers before opening non-seekable ZIP entry streams
• retain the existing 8 MiB fallback for unknown, invalid, oversized, or throwing lengths
• keep IFile, IProcessorState, and ProcessorState unchanged
• add boundary and real ZIP-entry tests

The source changes are independent from #56185. Performance was measured on top of its exact head.

Performance

Windows x64 Release ReadyToRun measurements used initialized dotnet new console --no-restore operations.

Delta from #56185

Metric	#56185	This change	Difference
Reported allocation	40,011,904 B	23,264,064 B	-41.86%
Peak heap	25.06 MB	5.89 MB	-19.17 MB
Promoted memory	16.40 MB	3.72 MB	-12.68 MB
GC count	2	1	-1
Console latency	227.53 ms	225.15 ms	0.99% paired improvement
Sampled CPU	330.50 ms/op	324.41 ms/op	-1.84%

The latency comparison used 15 alternating pairs; 11/15 candidate operations were faster with zero semantic mismatches.

Three-PR breakdown

PR	Change	Allocation result	Time result
#56163	Remove template-host logging DI construction	Host construction: 93,191 B to 9,801 B (-89.5%)	Host CPU: 15.15 ms to 4.44 ms; console creation 5.32% faster
#56185	Store the common trie child inline	Command allocation: 49,401,248 B to 40,047,344 B (-18.93%)	Console latency effectively neutral; list 4.08% faster
This PR	Size buffers from ZIP entry lengths	Command allocation: 40,011,904 B to 23,264,064 B (-41.86%)	Console creation 0.99% faster; sampled CPU 1.84% lower

Validation

• Microsoft.TemplateEngine.Core.UnitTests
  • Debug and Release
  • net11.0 and net481
  • 370 passed, 0 failed per run
• Microsoft.TemplateEngine.Edge.UnitTests
  • Debug and Release
  • net11.0 and net481
  • 269 passed, 0 failed per run
• full Release ReadyToRun SDK build
• exact measured SDK trees differed only in ReadyToRun Abstractions, Core, and Edge assemblies
• all measured commands and semantic comparisons passed

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[Copilot]

🟡 Changes recommended

The new ZIP-entry unit test uses a single Stream.Read call assuming it fills the buffer, which is not guaranteed and can cause intermittent failures.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR improves template processing performance by sizing ProcessorState buffers based on known ZIP entry lengths (when available) before opening non-seekable entry streams, while preserving the existing 8 MiB default fallback for unknown/invalid lengths.

Changes:

• Introduces an internal IKnownLengthFile contract in TemplateEngine Abstractions and exposes it to Core/Edge (and their unit tests) via InternalsVisibleTo.
• Implements IKnownLengthFile for ZipFileFile so ZIP entry uncompressed length can be queried without opening the entry stream.
• Updates Orchestrator to opportunistically select a smaller processing buffer size when IKnownLengthFile.Length is valid and below the default, and adds focused unit coverage (including ZIP-entry coverage).

File summaries

File	Description
test/TemplateEngine/Microsoft.TemplateEngine.Edge.UnitTests/ZipFileFileTests.cs	Adds ZIP-entry length validation coverage for ZipFileFile implementing known-length behavior.
test/TemplateEngine/Microsoft.TemplateEngine.Core.UnitTests/OrchestratorTests.cs	Adds unit tests covering buffer-size selection behavior for known/invalid/throwing lengths.
src/TemplateEngine/Microsoft.TemplateEngine.Edge/Mount/Archive/ZipFileFile.cs	Implements IKnownLengthFile by exposing ZIP entry uncompressed length.
src/TemplateEngine/Microsoft.TemplateEngine.Core/Util/Processor.cs	Makes DefaultBufferSize accessible internally for orchestrator buffer-size decisions.
src/TemplateEngine/Microsoft.TemplateEngine.Core/Util/Orchestrator.cs	Uses IKnownLengthFile.Length to choose a smaller buffer size before opening non-seekable streams.
src/TemplateEngine/Microsoft.TemplateEngine.Abstractions/Mount/IKnownLengthFile.cs	Adds internal interface for files that can reliably report length.
src/TemplateEngine/Microsoft.TemplateEngine.Abstractions/Microsoft.TemplateEngine.Abstractions.csproj	Grants friend access to internals for Core/Edge and their test assemblies.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Lite