Add exec channel request support (RFC 4254 §6.5)

[GlassOnTin]

Summary

Adds requestExec(command: String): Boolean to the SshSession interface and SessionChannel implementation, enabling remote command execution on session channels.

• Follows the same pattern as requestShell() — uses sendChannelRequest("exec", ...) with the existing ChannelRequestExec Kaitai definition
• Only one of requestShell, requestExec, or subsystem requests may succeed per session channel (per RFC 4254 §6.5)
• Integration tests verify stdout and stderr output from executed commands

Changes

• SshSession.kt — added requestExec(command: String): Boolean to the interface
• SessionChannel.kt — implementation using ChannelRequestExec protocol message
• SshClientIntegrationTest.kt — two integration tests:
  • Execute command and read stdout
  • Execute command and read stderr

Test plan

• ./gradlew :sshlib:test — integration tests pass against OpenSSH testcontainer
• Verify exec produces correct stdout output (echo hello-exec)
• Verify exec produces correct stderr output (echo error-output >&2)

[kruton]

You also have to run ./gradlew metalavaGenerateSignature since you're adding an API.

[kruton]

Run ./gradlew metalavaGenerateSignature

[GlassOnTin]

Done — ran metalavaGenerateSignature and pushed the updated api.txt. Only requestExec added to both SshSession and SessionChannel.