[rust-launcher] Replace expect()/unwrap() calls in production code with proper error handling

[barakeinav1]

Context

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

Problem

There are 6 expect()/unwrap() calls in non-test production code in crates/tee-launcher/src/main.rs that will panic instead of returning a proper error. These should use the ? operator with appropriate LauncherError variants.

Locations

Line	Code	Risk
307	.expect("bearer token received from docker auth is a valid header value")	High -- panics if registry returns malformed token
475	.expect("is valid digest")	High -- panics if docker inspect returns unexpected output
173	.expect("tee config serializes to TOML")	Medium -- unlikely but possible
144	.expect("re-serializing a toml::Table always succeeds")	Medium
504	.expect("port list is serializable")	Medium
138	.expect("image digest file has a valid path")	Low -- hardcoded constant

Suggested Fix

Convert each to ? with a proper error variant. Example for line 307:

// Before:
.expect("bearer token received from docker auth is a valid header value");

// After:
.map_err(|_| LauncherError::RegistryAuthFailed("invalid bearer token format".to_string()))?;

New error variants may be needed for cases that don't fit existing ones.