fix: replace *http.Request in log field with JSON-safe fields

[seeforschauer]

Change Description

Replace raw *http.Request struct in logrus field with individual JSON-serializable fields (method, url, contentType).

*http.Request contains GetBody func() (io.ReadCloser, error) which encoding/json cannot marshal. When mev-boost runs with -json flag, logrus JSONFormatter fails to serialize the field and silently drops the log entry.

Root Cause

server/register_validator.go:51 passed the full request struct:

log.WithFields(logrus.Fields{
    "request": req,  // contains func() — breaks json.Marshal
}).Debug(...)

Fix

Extract the useful fields instead:

log.WithFields(logrus.Fields{
    "method":      req.Method,
    "url":         req.URL.String(),
    "contentType": req.Header.Get("Content-Type"),
}).Debug(...)

Fixes #894.

Testing

• go build ./... — passes
• go test ./server/... — all pass

[seeforschauer]

Closing — duplicate of #895 by @dom-nie which addresses the same issue. Apologies for not checking existing PRs first.