Fix Redis busy loading classification and stabilize telemetry tests

[RAprogramm]

Summary

• classify Redis BusyLoadingError as DependencyUnavailable while preserving retry metadata
• guard the tracing telemetry unit test to avoid spurious races under full feature builds and align serde_json expectations
• bump the crate to 0.20.8 and document the fixes in the changelog and README snippet

Testing

• cargo +nightly fmt --
• cargo clippy -- -D warnings
• cargo build --all-targets
• cargo test --all
• cargo doc --no-deps
• cargo deny check
• cargo audit

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https://chatgpt.com/codex/tasks/task_e_68d3aa5208c0832b8c334859a99c3e30