Wire getExecutionPayloadBid to the auctioneer, checking parent_hash/parent_root

[0w3n-d]

Issue: #489 (step 3 of 6)

Stacked on #503 — base branch is od/gloas-step2-envelope-broadcast. Retarget to develop once #501/#503 merge.

What this PR does

getExecutionPayloadBid needs to check whether the requested parent_root is currently live -- that state (SlotData.payload_attributes_map) only exists inside the single-threaded auctioneer actor, reached via AuctioneerHandle. This mirrors get_header's existing architecture rather than the direct-trait-injection pattern step 2 used (that pattern doesn't fit here: step 2's envelope construction only needed one already-submitted payload, not live per-slot head-tracking state).

• New Event::GetExecutionPayloadBid + AuctioneerHandle::get_execution_payload_bid, parallel to GetHeader.
• Context::get_execution_payload_bid: checks the request's parent_hash against payload_attributes_map, then checks that entry's parent_beacon_block_root matches the request's parent_root (the two must be compatible, i.e. from the same beacon state, per spec).
• The handler now calls through to the auctioneer instead of unconditionally returning NO_CONTENT.
• ExecutionPayloadBid/SignedExecutionPayloadBid type aliases added to helix_types.

What this PR deliberately does not do

Never actually serves a bid -- every request currently reports "no bid available" (NoBidPrepared, a valid spec response), whether or not the parent checks pass. Serving a real bid needs a Gloas-shaped payload store, which needs step 5's builder→relay submission wire format. max_execution_payment enforcement is likewise not implemented: there's no value/execution_payment split to enforce it against yet, and step 4 hasn't landed the preference store. Both are next steps once step 5 lands.

Tests

• get_execution_payload_bid (pure function, independent of the heavy Context type): unknown parent hash, parent root mismatch, missing parent_beacon_block_root, and matching parent (still no bid, by design) — all four report NoBidPrepared, proving the guards actually run rather than short-circuiting.

Reviewer checklist

• CI (lint, unit-test) is green
• Matches the linked issue/step
• No unexplained scope creep or unrelated files touched