[roll] Roll fuchsia [ffx][ssh] Migrate command builders to structured errors

Refactor the `ssh` library to replace generic `anyhow::Result` bounds
across all public command building and config endpoints.

- Defined `SshConfigError` in `config.rs` to handle value parsing and key
  retrieval misses natively, transitioning all inner dictionary
  operations onto structured results.
- Defined `SshCommandError` in `ssh.rs` using `thiserror` macros to
  cleanly unify `SshConfigError`, `ffx_config::ConfigError`, and
  ManageSshControlMasterError pipelines.
- Re-typed all public builders (`build_ssh_command`,
  `build_ssh_command_with_env`, etc.) to return the raw
  `Result<Command, SshCommandError>`.
- Purged `anyhow!` allocations from internal code pipelines and isolated
  test harnesses via explicit module-level reinjections.

Test: `fx test ffx_ssh_lib_test`

Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1598869
Original-Original-Revision: 28e5ca87d5b410cf525ec0e635f0ce69fc0315a2
GitOrigin-RevId: abbee4217d10a3f0c4ad5cb3103c72dd48a8ad99
Change-Id: I2d4f44554445c060f062f9cbae30055a9cd57fef
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1612193
Cr-Commit-Position: refs/heads/main@{#192747}