feat(server): add request-level logging via TraceLayer

[sjenning]

Summary

• Add tower_http::trace::TraceLayer to gRPC and HTTP services in the gateway multiplexer to log method, path, HTTP status, and latency for every request
• Health/readiness probe endpoints (/health, /healthz, /readyz) are logged at debug level to reduce noise
• No new dependencies — tower-http with the trace feature was already in the workspace

Fixes#892

Related Issue

#892

Changes

• crates/openshell-server/src/multiplex.rs: Wrap both inner services with TraceLayer in MultiplexService::serve(), add make_request_span and log_response helper functions

Testing

• cargo check -p openshell-server passes
• cargo test -p openshell-server — all 8 tests pass (unit + integration)
• Verified TraceLayer is exercised by existing multiplex and WebSocket tunnel integration tests

Checklist

• Code compiles without errors
• Existing tests pass
• No new dependencies added
• Follows project conventions (conventional commits, no AI attribution)

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[sjenning]

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