Fix PooledBufferStream.Reset() to reset Position property

[Copilot]

PooledBufferStream.Reset() was not resetting the Position property to 0, causing position state to leak across pooled stream reuses.

Changes

• PooledBufferStream.cs: Added Position = 0; to Reset() method
• PooledBufferStreamTests.cs: Added test Reset_WithPosition_ResetsPositionToZero validating position resets for various initial values

Example

var stream = PooledBufferStream.Rent();
stream.SetLength(100);
stream.Position = 50;

PooledBufferStream.Return(stream);  // Calls Reset() internally

var recycled = PooledBufferStream.Rent();  // Gets same instance
// recycled.Position is now 0 (was 50 before fix)

Fixes #9274

Warning

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>PooledBufferStream doesn't reset the position when reset</issue_title>
<issue_description>In PooledBufferStream, the position is not set to 0 when calling Reset():

orleans/src/Orleans.Serialization/Buffers/Adaptors/PooledBufferStream.cs

Lines 95 to 105 in 970053a

	public void Reset()
	{
	foreach (var buffer in _segments)
	{
	Pool.Return(buffer);
	}
	_segments.Clear();
	_length = 0;
	_capacity = 0;
	}

It can cause issues when recycling a stream where the stream position is used. The position should be set to 0, and we should add a test to validate this behavior.
</issue_description>

<agent_instructions>Assign the PR to benjaminpetit</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes PooledBufferStream doesn't reset the position when reset #9743

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