feat(mev): relax SendEndOfBlockBundle validation

[vaigay]

What

Removes the three client-side guards on the end-of-block path so the caller, not the library, decides where a bundle goes.

Client.SendEndOfBlockBundle:

• dropped the sender-type allow list (Titan/Quasar) that returned ErrMethodNotSupport
• dropped the flashbotKey == nil guard that returned ErrMissingPrivKey. postBundle already attaches X-Flashbots-Signature only when a key is present, so a keyless client now simply sends an unsigned request

BloxrouteClient.SendEndOfBlockBundle:

• dropped the ReplacementSeqNumber != nil rejection. The field has no bloXroute equivalent and is now silently dropped

The len(req.TargetPools) == 0 check stays — both builders require the field.

Also makes SendEndOfBlockBundleRequest.ReplacementUUID a *string, so an unset UUID is distinguishable from an empty one and stays out of the JSON payload.

Breaking API notes

Two symbols published in v0.11.30 are removed because nothing uses them any more:

• BundleSenderType.SupportsEndOfBlockBundle() and its endOfBlockBundleSenderTypes map — keeping a predicate that no code path enforces would be a trap
• ErrReplacementSeqNumberNotSupport

And ReplacementUUID changes type from string to *string. Happy to keep any of these if a downstream service already depends on them.

Testing

• go build ./...
• go test -race ./pkg/mev/ — pass
• golangci-lint run --config=.golangci.yml ./pkg/mev/... — clean on both changed files (the pre-existing gosec/modernize issues in other pkg/mev files are untouched)

There is no test file for the end-of-block sender; the paths need a live builder endpoint, like the other bundle-sender tests.