fix(rpc): avoid rebuilding mempool index per transaction in getrawmempool

[syszery]

Motivation

Closes #10587

Fix performance issue in getrawmempool by avoiding repeated reconstruction of the mempool index for each transaction.

Implements the optimization suggested in the issue.

Solution

Move construction of transactions_by_id to the RPC caller and pass it into from_verified_unmined_tx, as outlined in the issue.

Tests

Existing property tests were updated to reflect the changed function signature. All tests pass locally.

AI Disclosure

• No AI tools were used in this PR
• AI tools were used: ChatGPT was used to draft the PR description and polish text.

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

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[syszery]

As a side note, I think we can optimize this further by removing .cloned() on the dependencies() HashSet. Since we already have empty_set defined above, maybe we can use .unwrap_or(&empty_set).iter().map(...). I will test this later.