fix(gcm): reset AEAD packet/nonce counters during key exchanges (#165)

[GT-610]

Re-applies upstream vicajilau#166 that was lost during the local 'Fix AES-GCM padding' commit (4518047).

AES-GCM AEAD ciphers must use a dedicated invocation counter that restarts at 0 on every NEWKEYS / key exchange, rather than the transport packet sequence number (_localPacketSN/_remotePacketSN) which keeps growing across rekeys. Using the SN as the GCM nonce causes 'message authentication code incorrect' failures against OpenSSH servers configured with aes256-gcm@openssh.com / aes128-gcm@openssh.com.

Changes:

• Add _localAeadPacketCount / _remoteAeadPacketCount fields.
• Use them as the AEAD sequence (with post-increment) in _sendAeadPacket and _consumeAeadPacket.
• Reset both counters to 0 in _applyLocalKeys / _applyRemoteKeys when an AEAD cipher is selected.

Refs: lollipopkit/flutter_server_box#1193

Summary by CodeRabbit

• Bug Fixes
  • 修复了 SSH 连接在使用 AEAD 加密方式（如 GCM、ChaCha20-Poly1305）时的包序号处理问题。
  • 在密钥更新后，AEAD 方向计数会正确重置，减少因序号不同步导致的传输异常或连接失败。

[coderabbitai]

📝 Walkthrough

概览

在 SSHTransport 中为 AEAD 加密路径（GCM / ChaCha20-Poly1305）新增独立的本地/远端数据包计数器 _localAeadPacketCount 与 _remoteAeadPacketCount，替换原先复用的通用序号，并在每次密钥轮换时将其重置为 0。

变更

AEAD 独立计数器

Layer / File(s)	摘要
字段声明与收发计数器使用 lib/src/ssh_transport.dart	新增 _localAeadPacketCount 与 _remoteAeadPacketCount 两个字段（初始值 0）；_sendAeadPacket 传入 _localAeadPacketCount++，_consumeAeadPacket 传入 _remoteAeadPacketCount++，使 AEAD nonce 计数与通用 packet SN 解耦。
密钥应用时重置计数器 lib/src/ssh_transport.dart	在 _applyLocalKeys 和 _applyRemoteKeys 的 AEAD 分支中分别将对应计数器归零，确保每次密钥轮换后 AEAD sequence 从 0 重新开始。

---

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@lib/src/ssh_transport.dart`:
- Around line 1039-1043: The rekey reset logic in the AEAD path only clears
`_localAeadPacketCount` when that branch is reached, but the server-side
`NEWKEYS` flow does not go through `_handleMessageKexReply()`. Update the server
rekey handling so `_localAeadPacketCount` is reset whenever new keys are
activated, using the same reset point as the client path in `ssh_transport.dart`
to keep AEAD nonce sequencing aligned after rekey.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: d831aa14-594f-4843-a9eb-c5f0145fe9aa

📥 Commits

Reviewing files that changed from the base of the PR and between aad680b and a5b1cda.

📒 Files selected for processing (1)

• lib/src/ssh_transport.dart

📜 Review details

🔇 Additional comments (4)

lib/src/ssh_transport.dart (4)

213-216: LGTM!

---

376-383: LGTM!

---

747-754: LGTM!

---

1109-1113: LGTM!