[rust-launcher] Add dstack/RTMR3 failure path tests

[barakeinav1]

Context

Follow-up from PR #2326 (port node launcher to Rust). Tracked in #2598.

Problem

The Python launcher had 4 tests covering dstack/RTMR3 failure paths. The Rust launcher has zero.

Python tests that existed:

• test_extend_rtmr3_nontee_skips_dstack -- NONTEE doesn't call dstack
• test_extend_rtmr3_tee_requires_socket -- TEE fails if socket missing
• test_extend_rtmr3_tee_getquote_fail -- GetQuote RPC failure handled
• test_extend_rtmr3_tee_emitevent_fail -- EmitEvent RPC failure handled

Action

Add tests covering:

• emit_event() errors propagate as DstackEmitEventFailed
• NONTEE mode skips dstack interaction entirely
• TEE mode with unreachable socket produces clear error

May require mocking the dstack_sdk::DstackClient or extracting it behind a trait.