fix/non-determinism-in-aggregator-order

[mchain0]

with a multimodal tie (e.g. two distinct values each appear the same number of times, and that number still satisfies modeHasQuorum for the chosen quorum type), different processes can legitimately build modes in different orders and return different modes[0]. that breaks the expectation that all honest nodes compute the same aggregated value for the same observation multiset.

fix: after collecting tied modes, choose one deterministically - by ordering and returning the observation for the smallest SHA.

TDD test approach: the test calls mode 400 times with the same slice and requires every result to match the first under proto.Equal, before the fix it pretty quickly failed: iteration 13: mode() must pick the same tied winner on every call, and after the fix it passes.

[github-actions]

👋 mchain0, thanks for creating this pull request!

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[github-actions]

📊 API Diff Results

No changes detected for module github.com/smartcontractkit/chainlink-common

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