fix(core): replace expect() with proper error handling

[lostmygithubaccount]

Summary

• Replace .expect() with Result return types in all three wire format build_request functions (completions, responses, messages) to prevent panics in library code
• Propagate errors through wire_formats::build_request dispatch and all client.rs call sites
• Add Python unit tests for wire format validation, message building, and health check error paths (5 → 10 unit tests)

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