walletrpc: allow conf_target=1 in EstimateFee

[starius]

Change Description

Previously walletrpc.EstimateFee used to fail if conf_target is 1.
This was discussed before in #9611

In this PR value 1 is allowed. An itest is added.

Steps to Test

make itest icase='estimate_fee'

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[gemini-code-assist]

Summary of Changes

Hello @starius, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the walletrpc.EstimateFee RPC method to allow a confirmation target of 1, addressing a previous limitation where such requests would fail. The change includes modifying the RPC handler to remove the explicit rejection of conf_target=1 and introduces a new integration test to validate this updated behavior, ensuring the API functions as expected for immediate confirmation targets.

Highlights

• API Behavior Change: The walletrpc.EstimateFee RPC method now permits a conf_target (confirmation target) of 1, which was previously explicitly rejected. This allows users to estimate fees for transactions aiming for confirmation in the very next block.
• Improved Error Messaging: The error message returned when conf_target is 0 has been updated to reflect that the target must be 'greater than 0', aligning with the new allowance for conf_target=1.
• New Integration Test: A new integration test, testEstimateFee, has been added to comprehensively verify the updated EstimateFee RPC behavior. It specifically tests for successful fee estimation when conf_target is 1 and confirms that conf_target of 0 still correctly triggers an error.

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[gemini-code-assist]

Code Review

This pull request correctly allows a confirmation target of 1 in walletrpc.EstimateFee and adds an integration test to verify the behavior. The changes are straightforward and well-tested. I've added a couple of suggestions: one to improve input validation in EstimateFee to also handle negative confirmation targets, and another to refactor the new test into a table-driven format for better maintainability and to include a test case for the improved validation.