Use encode callback instead of renting a buffer to write to in DSAKeyFormatHelper

[vcsjones]

This is a place I missed when adopting the new callback-based Encode on AsnWriter in #106728. This avoids a rent and a copy of data.

[Copilot]

PR Overview

This pull request updates the DSAKeyFormatHelper to use the new callback-based Encode API on AsnWriter instead of renting a buffer and copying data.

• Replaces direct buffer renting and TryEncode with a callback that writes a BitString directly to the writer
• Removes obsolete error handling via Debug.Fail and exception throwing on buffer encoding failure

Changes

File	Description
src/libraries/Common/src/System/Security/Cryptography/DSAKeyFormatHelper.cs	Converted buffer-based encoding to use AsnWriter's callback-based Encode, eliminating CryptoPool usage

Copilot reviewed 1 out of 1 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

src/libraries/Common/src/System/Security/Cryptography/DSAKeyFormatHelper.cs:277

• Ensure that the updated callback properly handles encoding failures. In the previous code, an explicit Debug.Fail and exception were triggered when TryEncode failed; please verify that inner.Encode will throw an exception or otherwise signal failure if encoding does not succeed.

inner.Encode(writer, static (writer, encoded) => {

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