Add Stateful ssh_session for Persistent SSH Connections

[jabbate19]

What type of PR is this?

/kind feature
/kind eldritch-function

What this PR does / why we need it:

Introduce pivot.ssh_session(target, port, username, password, key, key_password, timeout) -> <ssh_session> — the first ForeignValue object with methods in Eldritch. It solves the per-call auth-spam problem of ssh_exec by holding a single russh transport open for many channel_open_session+exec calls.

<ssh_session>.exec(command) returns Dict{stdout, stderr, status} identical to ssh_exec. <ssh_session>.close() disconnects the transport and is idempotent; subsequent exec() raises "session closed".

Implementation:

• implants/lib/eldritch/stdlib/eldritch-libpivot/src/lib.rs: add ssh_session signature to PivotLibrary trait
• implants/lib/eldritch/stdlib/eldritch-libpivot/src/std/mod.rs: wire factory, add pub mod ssh_session_impl
• implants/lib/eldritch/stdlib/eldritch-libpivot/src/std/ssh_session_impl.rs (new): SshSessionHandle = Arc<Mutex<Option>>; eager connect with timeout wrapper, take-restore across await to avoid holding lock, custom Debug that only prints target/port/connected (no secret leak), async helpers for tokio tests (wrap_for_test, exec_async, close_async, is_closed), mock sshd that supports multiple execs per connection with rising start-up guard (wait_until_listening)
• implants/lib/eldritch/stdlib/eldritch-libpivot/src/fake.rs: FakeSshSessionHandle + FakeSshSessionLibrary (exec/close) with closed-flag via spin::Mutex; PivotLibraryFake::ssh_session returns Foreign handle
• implants/lib/eldritch/stdlib/eldritch-libpivot/Cargo.toml: add spin optional dep for fake_bindings
• implants/lib/eldritch/eldritch/src/bindings_test.rs: fix registration-order bug where with_context re-registered Std pivot over fake (chain with_fake_agent after with_context); add file bindings drift (list_named_pipes, read_named_pipe, tmp_dir); add pivot.ssh_session to expected bindings; add test_ssh_session_object_methods, test_ssh_session_fake_exec_and_close, test_ssh_session_fake_reuse_multiple_execs

Tests: cargo test -p eldritch-libpivot --lib 48 passed (6 new),
cargo test -p eldritch --features fake_bindings 17 passed

Docs: user guide update for pivot.ssh_session is out of scope for this commit per repo conventions.

Which issue(s) this PR fixes:

Fixes #2003