Add object store observability wrapper

[thorrester]

Object Store Observability

Depends on merged #297.

Summary

• Adds an ObjectStoreSpanLayer wrapper for configured Delta object stores so direct object-store operations emit Phase 0 request spans and metrics.
• Instruments list, list_with_delimiter, head, get, get_range, put, delete, and copy with operation, backend, status, path kind, hashed path identity, range metadata, and byte counts.
• Classifies object paths as delta_log, parquet_data, checkpoint, or unknown without attaching full object paths by default.
• Extends CachingStore so cached hits, cache misses, and delegated direct calls participate in the same object-store span and metric vocabulary.
• Adds colocated tests for path classification, parquet footer range detection, and wrapper delegation behavior.

Verification

• make lints failed on an outside-scope dead-code warning in crates/scouter_dataframe/src/parquet/tracing/engine.rs for DELTA_SNAPSHOT_REFRESH.
• cargo test -p scouter-dataframe --all-features -- --nocapture --test-threads=1 passed.
• Python verification skipped because this change does not touch PyO3-exposed types or Python behavior.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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