Preserve manifest input streams when writing manifests

[dtivel]

Fixes #14881.

Summary

Ensure ManifestWriter.WriteManifest leaves the manifest's caller-owned input stream open after writing.

PR #9983 began disposing the StreamReader, which also disposed the underlying manifest input stream. This caused subsequent calls to ManifestWriter.WriteManifest using the same manifest to fail with Stream was not readable.

The explicit StreamReader arguments preserve the previous encoding, BOM detection, and buffer-size behavior while setting leaveOpen: true.

Compatibility

This restores the stream-ownership behavior from before PR #9983. No ChangeWave is needed because this is a narrow regression fix and does not change the public API surface.

Tests

Added regression coverage that:

• writes the same manifest twice after rewinding its preserved input stream;
• verifies the input stream remains readable after each write;
• verifies both writes produce identical, non-empty output.

The regression test passes on both net11.0 and net472.

[Copilot]

Copilot review overview

Review tier: Lite
Findings: 1

New issues introduced by this change (1)

Severity	Finding
	src/​Tasks/​ManifestUtil/​Util.cs — WriteFile(string, Stream) now uses leaveOpen: true, so it no longer disposes the…

What changed in this PR

This PR fixes a regression in the manifest utilities where writing a manifest could dispose the manifest’s preserved (caller-owned) input stream, breaking subsequent reuse of the same Manifest instance.

Changes:

• Update Util.WriteFile(string, Stream) to construct a StreamReader with leaveOpen: true so disposing the reader doesn’t close the supplied stream.
• Add a regression unit test that writes the same manifest twice after rewinding its preserved input stream and verifies both outputs match and the input stream remains readable.

File	Description
src/​Tasks/​ManifestUtil/​Util.cs	Preserves caller-owned streams by leaving the input stream open when reading it via StreamReader.
src/​Tasks.UnitTests/​ManifestWriter_Tests.cs	Adds regression coverage to ensure ManifestWriter.WriteManifest doesn’t dispose preserved input streams and that repeated writes are consistent.

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