Handle incompatible discovery fork IDs

[flcl42]

Changes

• Defer discv4 peer admission while a refreshed ENR fork ID is required or already pending.
• Remove discv4 discovery nodes when a refreshed signed same-node ENR fails fork-id validation.
• Avoid publishing bare-node health updates from handled ENR responses before the refreshed ENR has been validated.
• Add regression coverage for incompatible refreshed ENRs and pending ENR refreshes followed by sequence-less pings.

Types of changes

• Bugfix (a non-breaking change that fixes an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Optimization
• Refactoring
• Documentation update
• Build-related changes
• Other: Description

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Notes on testing

Targeted discovery/fork-id regression coverage passed. The broader discovery project still has an unrelated E2E discovery failure on this branch.

Documentation

Requires documentation update

• Yes
• No

Requires explanation in Release Notes

• Yes
• No

Remarks

None.