feat: DI-1192 client-visible rejection messages via SSH auth banner

[MatthewwTN]

Clever Coding Standards Agreement

• Author and Review Statement, "We agree this code adheres to our Clever Global Coding Standards and other applicable Coding Standards"

Linear

DI-1192

Overview

• As part of the static IP migration (DI-1192), clever-sftp will reject logins from districts tagged use_sftp2_host_only when they connect to the legacy us-west-1 host. Today a rejected login only shows the client's generic "Permission denied, please try again." — districts get no explanation of why they can no longer connect, which will drive support load during the sftp2.clever.com rollout.
• The server's auth-callback error string never reaches the client (SSH only transmits a failure flag); the protocol's mechanism for messaging during a rejected login is the auth banner, which OpenSSH/WinSCP/FileZilla print by default.
• This adds an opt-in, backwards-compatible API: NewManagedServerWithError accepts a generator returning (ServerDriver, error); returning a nil driver with a *RejectionError sends its message to the client as an auth banner via ssh.BannerError. All other denials (nil driver, plain/nil error) behave exactly as today. NewManagedServer is unchanged, so the other consumer (lockbox-sftp) is unaffected.
• Bumps golang.org/x/crypto to v0.40.0 (matching clever-sftp) — ssh.BannerError did not exist in the previously pinned 2022 version.

Testing

• go build and the package test suite pass.
• New table-driven test TestRejectionToSSHError covers: banner from a RejectionError, banner from a wrapped RejectionError, trailing-newline normalization, and generic denial for nil/plain errors.
• End-to-end banner display will be verified against clever-dev when clever-sftp is wired up (companion PR: Clever/clever-sftp#126).

Rollout

• No behavior change for existing consumers until they opt in via NewManagedServerWithError.
• Merge order: this PR merges first, then Clever/clever-sftp#126 pins the new commit and adopts the API.

Rollback

• Revert the commit; consumers that haven't adopted NewManagedServerWithError are unaffected. If clever-sftp has already adopted it, re-pin its go.mod to the prior pseudo-version as part of the revert.