[6.0] Allow zero byte reads on the test server response body

[Tratcher]

YARP adopted the zero-byte-read pattern to reduce memory usage on long lived connections. However, we've found several existing streams in the framework that do not support that pattern.

Description

In-memory web app testing using TestServer is a pattern we recommend to make tests more reliable. Customers can set up YARP to proxy to an in-memory TestServer instance to test multiple applications together. However, when YARP adopted the zero-byte-read pattern, TestServer didn't support that and the scenario broke.

There is no known workaround for 6.0. The issue was partially fixed in 7.0 via new Stream Memory overloads.

Fixes #41692

Customer Impact

YARP customers are currently blocked from testing by proxying to in-memory apps via TestServer.

Regression?

• Yes
• No

This worked with YARP 1.0 before zero-byte-reads were adopted in 1.1.

Risk

• High
• Medium
• Low

This is a testing component.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

[ghost]

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Otherwise, please add tell-mode label.

[ghost]

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[wtgodbe]

Seems reasonable to me