fix: reward callback failure unlock

[dongcool]

Summary

This PR prevents a validator from remaining locked if the reward balance refresh callback receives a failed external promise.

Problem

epoch_update_rewards() sets the validator into executing state before calling the external staking pool. The callback previously used #[callback], which panics
before entering the function body if the external call fails.

In that failure case, the validator never reaches the normal post_execution() path, so it can remain stuck in executing = true and block future operations for that
validator.

Fix

Change validator_get_balance_callback to use #[callback_result].

On success, the existing reward update logic is unchanged. On failure, the callback clears the validator execution flag and returns without updating rewards.

Testing

CI passed:

• test
• clippy and fmt

[linguists]

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📥 Commits

Reviewing files that changed from the base of the PR and between 7620e4e and a759814.

📒 Files selected for processing (3)

• contracts/linear/src/epoch_actions.rs
• contracts/linear/src/validator_pool.rs
• tests/__tests__/linear/epoch-action-failure.ava.ts

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[linguists]

Most of the functions in the current codebase use the is_promise_success() function to check callback success/failure. The current approach in the PR also works.

[dongcool]

Keeping the callback_result approach here since it handles failed promise results directly and releases the validator lock for retry