refactor: replace gliderlabs/ssh with golang.org/x/crypto/ssh (#38837)

Migrate away from this thin ssh wrapper module while adding more test
coverage.

Removes `sessionPartial`, which hand-copied the layout of a private
`gliderlabs/ssh` struct and reinterpreted a pointer to it via
`reflect.UnsafePointer` to reach the permissions of the authenticated
connection. The layout is unchecked, so an upstream field reorder would
mismatch silently.

The builtin server only needs the session channel with `exec` and
`shell`. Serving those on `x/crypto` drops the hack and the dependency,
since `PublicKeyCallback` returns permissions per key and `x/crypto`
assigns them only after verifying the signature.

Two benign behavior changes:

1. Internal session handler errors report exit status 1 rather than 0,
so a client no longer reads a failure as success.
1. An unusable host key is fatal at startup instead of being replaced by
an ephemeral one that would trigger an error at the client.

go.mod

@@ -44,7 +44,6 @@ require (
 	github.com/felixge/fgprof v0.9.5
 	github.com/fsnotify/fsnotify v1.10.1
 	github.com/getkin/kin-openapi v0.145.0
-	github.com/gliderlabs/ssh v0.3.8
 	github.com/go-chi/chi/v5 v5.3.1
 	github.com/go-chi/cors v1.2.2
 	github.com/go-co-op/gocron/v2 v2.22.0
@@ -133,7 +132,6 @@ require (
 	github.com/STARRY-S/zip v0.2.3 // indirect
 	github.com/andybalholm/brotli v1.2.1 // indirect
 	github.com/andybalholm/cascadia v1.3.4 // indirect
-	github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be // indirect
 	github.com/aws/aws-sdk-go-v2 v1.43.1 // indirect
 	github.com/aws/aws-sdk-go-v2/internal/configsources v1.4.32 // indirect
 	github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.7.32 // indirect