fix(object-store): make bucket error-rate respond to successes

[shikhar]

Summary

• switch bucket error-rate updates to request-level EMA so successes decrease penalty and failures increase it
• retain existing time-decay and circuit-breaker behavior
• add regression coverage for high-success traffic after an error and update the zero-elapsed success expectation

Testing

• cargo +nightly fmt
• cargo clippy --all-features --all-targets -- -D warnings --allow deprecated
• cargo nextest run test_success_without_elapsed_time_reduces_error_penalty test_success_traffic_drives_error_rate_down test_error_rate_simple test_error_rate_accumulation test_error_rate_decay

Closes #87

[greptile-apps]

Greptile Summary

Switched bucket error-rate tracking from time-decay-only to proper request-level EMA where successes actively reduce error penalty and failures increase it.

Key changes:

• Successes now multiply error rate by (1 - α) ≈ 0.985, actively driving it down
• Failures apply the same decay factor then add α, properly penalizing errors
• Time-decay behavior preserved through initial error_rate(now) call
• Circuit breaker logic unchanged

Test coverage:

• Updated test_success_without_elapsed_time_reduces_error_penalty to verify successes reduce penalty even with zero latency
• Added test_success_traffic_drives_error_rate_down to verify 1000 successes suppress error rate to <5%
• Existing tests validate error accumulation and decay behavior remain sound

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• Clean implementation of the EMA formula with mathematically sound logic. Comprehensive test coverage validates both the new success-driven error reduction and existing failure accumulation behavior. Code follows Rust best practices and project style guidelines.
• No files require special attention

Important Files Changed

Filename	Overview
src/object_store/stats.rs	Implements request-level EMA for error rates - successes now actively decrease error penalty by factor (1-α), failures increase it. Includes comprehensive test coverage for the new behavior.

_{Last reviewed commit: c57bfa0}