fix(earn-quest): use per-asset decimals in convert_reward_amount (#1947)

[EmeditWeb]

Closes #1947

Summary

convert_reward_amount hardcoded a 7-decimal assumption (10_000_000) for every conversion, silently producing incorrect results for reward tokens with a different decimal count. This PR makes the conversion use each asset's actual decimals.

Changes

• src/lib.rs: convert_reward_amount now looks up each asset's actual decimals via the SEP-41 TokenClient::decimals() interface and uses the oracle price's reported decimals. The conversion scales by 10^(from_decimals + price_decimals - to_decimals) instead of the hardcoded constant.
• src/oracle.rs: get_aggregated_price now reports the decimal precision of the underlying price sources instead of a fixed 7.
• tests/test_reward_conversion.rs: new integration tests covering 6/7/18-decimal assets, non-7 price quotes (8 decimals), Stellar asset contracts, mixed decimals with non-integer prices, legacy 7/7 behavior preservation, and the AmountTooLarge overflow guard.

Verification

• New test suite: 9/9 pass
• Full contract test suite: all pass
• cargo fmt --check and cargo clippy --all-targets --all-features -- -D warnings clean

[RUKAYAT-CODER]

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