fix(validator): reload Web3Signer keys after same-size file updates

[questfever]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

The Web3Signer key-file watcher currently uses a change in file size as a proxy for a change in file contents.

Validator public keys have a fixed width. Replacing one key with another while preserving the number of keys and formatting therefore leaves the file size unchanged. Although fsnotify reports the write, Prysm skips re-reading the file and keeps the previous key set in providedPublicKeys.

For example, replacing an equal-length key A with key B does not reload the configured validators until another size-changing update occurs, the watcher is reinitialized, or the validator client is restarted.

This PR removes the file-size gate and re-reads the key file after relevant watcher events. The existing key comparison prevents providedPublicKeys and account-change subscribers from being updated when the parsed keys have not changed.

A regression test performs an in-place overwrite from key A to an equal-length key B, verifies that the file size remains unchanged, and asserts that SubscribeAccountChanges receives key B.

Which issue(s) does this PR fix?

Related to #17322.

This PR fixes only the same-size key-file reload bug described there and does not close the broader key-source management proposal.

Other notes for review

• Added a regression test for an equal-size in-place key-file update.
• Verified that the test times out before the fix.
• Verified that the test receives key B after removing the size gate.
• go test ./validator/keymanager/remote-web3signer

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description with sufficient context for reviewers to understand this PR.
• I have tested that my changes work as expected and I added a testing plan to the PR description (if applicable).

[CLAassistant]

All committers have signed the CLA.