schannel: small simplifications

[bagder]

Refactor and simplify the Schannel code, primarily by reducing
duplicated buffer-management and credential-setup logic.

• split client certificate selection into get_client_cert() and SSPI
  credential acquisition into acquire_sspi_handle()
• introduce a struct sbuffer for encrypted/decrypted buffering
• Add ensure_encoding_size() and ensure_decoding_size() helpers to
  centralize buffer growth/realloc decisions
• Tighten variable scopes and tidy indentation/logging in the handshake
  and receive/decrypt loops.
• Update comments and adjusts some receive error-condition handling to
  better preserve buffered-data behavior.

[bagder]

augment review

[augmentcode]

🤖 Augment PR Summary

Summary: This PR refactors and simplifies the Schannel TLS backend, primarily by reducing duplicated buffer-management and credential-setup logic.

Changes:

• Splits client certificate selection into get_client_cert() and SSPI credential acquisition into acquire_sspi_handle().
• Introduces a shared struct sbuffer for encrypted/decrypted buffering (length/offset/buffer) and updates handshake/recv code to use it.
• Adds ensure_encoding_size() and ensure_decoding_size() helpers to centralize buffer growth/realloc decisions.
• Tightens variable scopes and tidies indentation/logging in the handshake and receive/decrypt loops.
• Updates comments and adjusts some receive error-condition handling for clearer state management.

Technical Notes: Centralizing buffer sizing reduces repeated arithmetic and makes Schannel record handling easier to reason about across handshake and data paths.

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[augmentcode]

Review completed. 3 suggestions posted.

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[Copilot]

Pull request overview

This PR refactors the Schannel TLS backend internals to simplify buffer management and tighten up local variable scoping, while also cleaning up indentation/comments and adjusting an error-handling path.

Changes:

• Replace separate encdata_* / decdata_* fields with a shared struct sbuffer for encrypted/decrypted buffers.
• Introduce ensure_encoding_size() / ensure_decoding_size() helpers and use them in handshake and receive paths.
• Minor cleanups: request-flag expression simplification, indentation fixes, reduced variable scopes, and a small recv error-path adjustment.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
lib/vtls/schannel_int.h	Introduces struct sbuffer and updates backend state to use it; moves the encdata_is_incomplete comment.
lib/vtls/schannel.c	Refactors handshake/recv buffer logic to use struct sbuffer and new ensure-size helpers; scope/indent fixes and a recv error-path tweak.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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[bagder]

augment review

[augmentcode]

Review completed. 2 suggestions posted.

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[bagder]

@aisle-analyzer give a look as well

[aisle-research-bot]

🔒 Aisle Security Analysis

✅ We scanned this PR and did not find any security vulnerabilities.
_{Aisle supplements but does not replace security review.}

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Analyzed PR: #20569 at commit b0e91a3