fix: make oracle fallback path panic-free and event-emitting

[Agbasimere]

Summary

Closes #544

This hardens the graceful degradation fallback path so oracle backup failures never panic mid-resolution.

What changed

• made OracleBackup::get_price return typed contract errors when both primary and backup fail
• removed panic-capable fallback behavior from the active graceful degradation path
• emit degradation events for both primary and backup oracle failures
• added persisted oracle degradation state tracking so timeout handling moves the oracle into a defined degraded state
• updated handle_oracle_timeout to record degraded status and emit a degradation event
• updated oracle health reporting so degraded status is surfaced explicitly

Why

The fallback path is the safety net when the primary oracle is unavailable. If that path panics or aborts, a market can get stuck during resolution. This change makes the path degrade gracefully instead:

• both-oracles-down returns Error::FallbackOracleUnavailable
• timeout handling records a defined degraded state
• degradation is observable through emitted events

Files changed

• contracts/predictify-hybrid/src/graceful_degradation.rs
• contracts/predictify-hybrid/src/lib.rs

Tests

cargo test -p predictify-hybrid graceful_degradation -- --test-threads=1
cargo test -p predictify-hybrid -- oracle_fallback --test-threads=1
cargo test -p predictify-hybrid --lib -- --test-threads=1

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