fix: add exponential recovery backoff

[nanookclaw]

Summary

Closes #192.

This changes Rust stream creation and recovery retries from a fixed interval default to exponential backoff with full jitter. recovery_backoff_ms remains the fixed interval when RetryStrategy::Fixed is selected, and becomes the base delay for RetryStrategy::ExponentialBackoffWithJitter; max_recovery_backoff_ms caps the computed exponential delay before jitter.

The same retry helper is used by gRPC stream creation, Arrow Flight stream creation, and Arrow supervisor recovery sleeps so the behavior stays consistent across both stream implementations. The default strategy is now exponential backoff with jitter, so wrapper SDKs that rely on the Rust core inherit the safer retry behavior without new dependencies.

Verification

• cargo fmt --manifest-path rust/sdk/Cargo.toml
• cargo check -p databricks-zerobus-ingest-sdk
• cargo check -p databricks-zerobus-ingest-sdk --features arrow-flight
• cargo test -p databricks-zerobus-ingest-sdk --lib
• cargo test -p databricks-zerobus-ingest-sdk --features arrow-flight --lib
• git diff --check