fix: restore count-based outlier quorum in price aggregation

[micahkendall]

Motivation

• The outlier-quorum logic was changed to use total weight remaining after filtering which allows a single dominant-weight source to survive outlier removal and determine the published price.
• The change restores multi-source integrity by requiring a majority of original sources (by count) to remain after outlier filtering before returning a price.

Description

• Replaced the weight-based quorum check in compute_value_in_usd with a count-based majority check using total_sources and candidate_prices.len() in src/price_aggregator/conversions.rs.
• Kept the weighted median and weighted averaging behavior for normal aggregation, but require candidate_prices.len() >= 1 + total_sources / 2 to proceed.
• Added a regression test value_in_usd_should_not_allow_single_dominant_weight_source_after_outlier_filtering to assert that a lone dominant-weight source cannot produce a price after outlier filtering.

Testing

• Ran cargo test -q value_in_usd_should_not_allow_single_dominant_weight_source_after_outlier_filtering which passed.
• Ran cargo test -q value_in_usd_should_not_report_price_if_sources_are_too_far_apart which passed.
• Ran cargo test -q value_in_usd_should_ignore_outlier_source which passed.

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Codex Task