fix(state): correct dequeue_children by_height index handling

[syszery]

Motivation

Fixes incorrect behavior in QueuedBlocks::dequeue_children where removing queued children incorrectly removed entire by_height buckets instead of only the matching block hash.

Closes #10597

Solution

Update dequeue_children to remove only the specific block hash from the by_height index, and only remove the height entry when no hashes remain.

Tests

Added a regression test that constructs a forked chain scenario where two queued blocks exist at the same height but belong to different parents. The test fails on the previous implementation and passes after the fix.

AI Disclosure

• No AI tools were used in this PR
• AI tools were used: assisted in drafting the test and initial fix structure

PR Checklist

• The PR title follows conventional commits format: type(scope): description
• The PR follows the contribution guidelines.
• This change was discussed in an issue or with the team beforehand.
• The solution is tested.
• The documentation and changelogs are up to date.

[claude]

Claude Code Review

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[natalieesk]

Thank you for this PR. We will take a look shortly.