ssh: use errors.As for PartialSuccessError unwrapping

[XananasX7]

ssh: use errors.As for PartialSuccessError unwrapping

When BannerError wraps a PartialSuccessError, the auth loop
failed to detect it. Use errors.As instead of a type assertion
so that wrapped PartialSuccessError values are recognised.

A new test TestBannerWrappingPartialSuccess is added to verify
the banner is delivered, the client completes the password second
factor, and the connection is established.

Updates golang/go#79809

[google-cla]

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[XananasX7]

Rewrote the commit with mehdiananas007@gmail.com as the author email — this is the address I used to sign the Google CLA. The cla/google check should now pass.

[gopherbot]

This PR (HEAD: 4fe8209) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/crypto/+/790420.

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/790420.
After addressing review feedback, remember to publish your drafts!

[XananasX7]

Updated the PR description to address the Gerrit bot feedback: removed all Markdown formatting (backticks, bold, code fences), wrapped all lines to ≤76 characters, and added 'Updates golang/go#79809' as the bug reference.

[gopherbot]

Message from Ian Lance Taylor:

Patch Set 2:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/790420.
After addressing review feedback, remember to publish your drafts!

[XananasX7]

Trimmed the commit message to plain text, ≤76 char lines, no markdown formatting — per Ian Lance Taylor's Gerrit review feedback. The fix itself is unchanged.

[gopherbot]

This PR (HEAD: 724988c) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/crypto/+/790420.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.