[release/9.0] Fix MultipartReaderStream synchronous read when using buffer offset

[github-actions]

Backport of #59360 to release/9.0

/cc @BrennanConroy

Fix MultipartReaderStream synchronous read when using buffer offset

Description

When synchronously reading a multipart section and using a non-zero offset value for the passed in buffer, we incorrectly ignored the offset and would write into the buffer starting at the beginning. This would overwrite previous data in the buffer which corrupts the user payload.

Fixes #59269

Customer Impact

Corrupted buffer data for a multipart section.

Regression?

• Yes
• No

Regressed in 9.0

Risk

• High
• Medium
• Low

Silly error around not using the buffer offset. Added missing test coverage.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

[dotnet-policy-service]

Hi @github-actions[bot]. This PR was just approved to be included in the upcoming servicing release. Somebody from the @dotnet/aspnet-build team will get it merged when the branches are open. Until then, please make sure all the CI checks pass and the PR is reviewed.

[dotnet-policy-service]

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