feat: implement Pyth oracle integration

[Happybello365]

Replace the pyth_client.rs stub with a full implementation:

• Define a Price struct (price, conf, expo, publish_time) matching the official Pyth Soroban SDK type, replacing the raw (i64,u64,i32,i64) tuple
• Add get_price_no_older_than to PythOracleInterface so the Pyth contract enforces staleness atomically on-chain (preferred resolution path)
• Keep get_price for permissive/test mode (max_staleness_seconds = u64::MAX)
• Update oracles.rs to re-export Price as PythPrice for backward compat, use get_price_no_older_than in fetch_pyth_price for production paths, and add secondary confidence validation via validate_price
• Feed ID remains configurable per market via OracleConfig.feed_id (64-char hex string decoded to BytesN<32> at call time)
• Update MockPythContract in oracles_test.rs to return Price struct and implement get_price_no_older_than with staleness enforcement
• Add test_pyth_integration.rs with dedicated integration tests covering:
  • Feed ID configurable per market and stored independently
  • get_price path (permissive mode)
  • get_price_no_older_than path (panics when stale)
  • validate_price staleness and confidence checks
  • resolve_with_pyth end-to-end outcome determination
  • No partial storage written on stale/failed resolution
  • Multiple oracle IDs are independent per market

Description

Type of Change

• Bug fix
• New feature
• Refactor / code cleanup
• Documentation update
• CI / tooling change
• Breaking change

Testing Done

Checklist

• Tests pass locally
• Documentation updated (if applicable)
• No breaking changes, or breaking changes are documented above

Related Issues

Closes #703

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