fix(p2p): skip UDP discovery whenever nodiscover is enabled

[gzliudan]

Proposed changes

This PR aligns discovery startup behavior with geth by making startup return early when nodiscover is enabled.

Motivation
Quick-test intermittently failed with address already in use caused by UDP discovery socket binding during nodiscover startup paths.

Impact
Nodes started with nodiscover no longer bind UDP discovery sockets, which removes intermittent port-collision failures and keeps behavior consistent with expected nodiscover semantics.

Types of changes

What types of changes does your code introduce to XDC network?
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• build: Changes that affect the build system or external dependencies
• ci: Changes to CI configuration files and scripts
• chore: Changes that don't change source code or tests
• docs: Documentation only changes
• feat: A new feature
• fix: A bug fix
• perf: A code change that improves performance
• refactor: A code change that neither fixes a bug nor adds a feature
• revert: Revert something
• style: Changes that do not affect the meaning of the code
• test: Adding missing tests or correcting existing tests

Impacted Components

Which parts of the codebase does this PR touch?
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• Consensus
• Account
• Network
• Geth
• Smart Contract
• External components
• Not sure (Please specify below)

Checklist

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• This PR has sufficient test coverage (unit/integration test) OR I have provided reason in the PR description for not having test coverage
• Tested on a private network from the genesis block and monitored the chain operating correctly for multiple epochs.
• Provide an end-to-end test plan in the PR description on how to manually test it on the devnet/testnet.
• Tested the backwards compatibility.
• Tested with XDC nodes running this version co-exist with those running the previous version.
• Relevant documentation has been updated as part of this PR
• N/A

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