Propagate optimistic head errors

[kediyalua]

Issue Addressed

Closes #10295

Also relates to #3822, which introduced the endpoint with this behaviour.

Proposed Changes

• POST /eth/v1/beacon/rewards/attestations/{epoch} no longer discards errors from
  BeaconChain::is_optimistic_or_invalid_head. The handler now propagates them with
  map_err(warp_utils::reject::unhandled_error)?, so a head whose execution status cannot be
  determined produces a server error instead of a response asserting
  execution_optimistic: false.

let execution_optimistic = chain
    .is_optimistic_or_invalid_head()
    .map_err(warp_utils::reject::unhandled_error)?;

• This makes the handler consistent with every other call site of
  is_optimistic_or_invalid_head in beacon_node/http_api (state_id.rs, block_id.rs,
  proposer_duties.rs, sync_committees.rs, and the eth/v1/node/syncing-style handler in
  lib.rs). The attestation rewards handler was the only one using unwrap_or_default().

Additional Info

• Behaviour is unchanged when is_optimistic_or_invalid_head() returns Ok(_): the endpoint still
  returns HTTP 200 with the same execution_optimistic value. The existing integration test
  test_beacon_attestation_rewards_fulu, which asserts execution_optimistic == Some(false), is
  unaffected.
• The error path is not covered by a test. It requires the fork choice race described in the
  is_optimistic_or_invalid_head doc comment (head block root pruned while syncing), which the
  test harness cannot easily force. I did not add a test rather than add one that does not exercise
  the path.
• No changes to the Beacon API schema or to any other endpoint.
• Verified locally: cargo check -p http_api --locked passes, and
  rustfmt --edition 2024 --check beacon_node/http_api/src/lib.rs reports no diff.
• Disclosure: this change was prepared with AI assistance; I reviewed the diff and checked it
  against the surrounding call sites.

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[michaelsproul]

sure