Fix minor issues and add a regress test

[yosuke-wolfssl]

This PR fixes minor issues addressed by static analysis.

• Fix IdentifyAsn1Key to free key on failure path
• Fix control flow in SendKexDhReply
• Remove dead code in SFTP_CreateLongName
• Fix resource management in wolfSSH_SFTP_RecvFSTAT
• Fix resource management in wolfSSH_SFTP_RecvRead (Windows path)
• Add a regress test for IdentifyAsn1Key

Issues: F-861, F-862, F-1273, F-1680 and F-1681

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR addresses several static-analysis findings around resource cleanup and control flow, and adds a regression unit test for IdentifyAsn1Key.

Changes:

• Fix key cleanup on ASN.1 decode failure paths in IdentifyAsn1Key.
• Fix KEX DH reply control flow and tighten SFTP buffer/resource management.
• Add a regression unit test with embedded DER test keys for IdentifyAsn1Key.

Reviewed changes

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

File	Description
tests/unit.c	Adds self-contained DER fixtures and a regression test covering IdentifyAsn1Key failure-path cleanup.
src/wolfsftp.c	Removes dead free and adds frees on early-return error paths to avoid leaks.
src/internal.c	Ensures key objects are freed on decode failures and corrects Curve25519/ECDH control flow.

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