Add fallback-path tests for Oracle source failures

[nanaf6203-bit]

Problem Statement

Oracle circuit-breaker trips in tests/integration_bridge_oracle.rs:233-249 but no ‘2-of-3’ feed outage test exists.

Why it matters

Reality: feeds go down intermittently; orchestration must degrade gracefully.

Technical Context

Test that fee aggregation falls back to TWAP when sources fail.

Expected Outcome

Aggregation reduces to TWAP and emits OracleFallbackToTwap event.

Acceptance Criteria

1. tests/oracle_feed_outage.rs covers 2-of-3 outage.
2. OracleFallbackToTwap event observed.
3. Kani harness.

Implementation Notes

Add TWAP computation module and tests.

Files or modules likely to be affected

contracts/oracle/src/lib.rs, tests.

Dependencies

#121 (TWAP) for fallback completion.

Difficulty level

MEDIUM.

Estimated effort

S (~1 day).

[blessme247]

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[Habibah371]

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can i tackle this issue?

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