propagate trace parse error in extract_sload_slots

[ashleychandy]

Description

extract_sload_slots previously called unwrap_or_default() on the geth
trace parse, silently treating non-default frame variants as empty traces.
This caused parse failures to surface as DetectionError::NotFound rather
than a distinct simulation error, making root cause analysis harder.

Changes

• Change extract_sload_slots return type to Result<Vec<(Address, B256)>, SimulationError>
• Replace unwrap_or_default() with explicit try_into_default_frame() error propagation
• Propagate error at call sites in approval and balance detectors via ?
• Fix structured logging in balance: move token to a tracing field

[gemini-code-assist]

Code Review

This pull request refactors extract_sload_slots to return a Result instead of a vector, replacing the previous unwrap_or_default fallback with explicit error propagation and warning logs when trace parsing fails. Callers in approval/mod.rs and balance/mod.rs have been updated to propagate these errors. No critical issues found.

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