Fix duplicate quick block fetching for the same hash

[Copilot]

Quick block fetching triggers multiple times for the same block hash when a block is imported via direct broadcast while an async quick fetch is pending. The forgetHash call clears f.announced, causing subsequent announcements to pass the len == 1 check and trigger redundant fetches.

Changes

• Add quickFetching map[common.Hash]struct{} to track in-flight async quick fetches
• Check quickFetching before triggering new quick fetch in notification handler
• Clear quickFetching entry only when async operation completes (success or error), not in forgetHash

// Before: only checked announced length
if f.fetchRangeBlocks != nil && len(f.announced[notification.hash]) == 1 {
    f.asyncFetchRangeBlocks(notification)
}

// After: also check if quick fetch already in progress
if f.fetchRangeBlocks != nil && len(f.announced[notification.hash]) == 1 {
    if _, ok := f.quickFetching[notification.hash]; !ok {
        f.quickFetching[notification.hash] = struct{}{}
        f.asyncFetchRangeBlocks(notification)
    }
}

Test

Added TestQuickBlockFetchingNoDuplicates that verifies fetchRangeBlocks is called only once when the same block is announced by multiple peers while an async fetch is pending.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Quick block fetching more than once for the same hash</issue_title> <issue_description>#### System information Geth version: geth version OS & Version: Windows/Linux/OSX Commit hash : (735f8ea4b70f52ad3da7282447a099d66083a569) Arguments:

Description

• Expected behaviour ...

• Actual behaviour ...

Backtrace

[backtrace]

When submitting logs: please submit them as text and not screenshots. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes bnb-chain/bsc#3405

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