[rust-launcher] Add retry/backoff behavior tests for registry manifest resolution

[barakeinav1]

Context

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

Problem

Neither the Python nor Rust launcher tests the actual retry timing/sequence for registry manifest resolution. The backon exponential backoff configuration (factor 1.5, max 60s, configurable max attempts) is untested.

Action

• Test that transient failures are retried up to rpc_max_attempts
• Test that permanent failures exhaust retries and return error
• Test that backoff intervals increase (optional, low priority)

Low priority.