`addPayloadEnvelope` should be idempotent for API/gossip race

## Context

In `PayloadEnvelopeInput.addPayloadEnvelope()`, a second call throws `"Payload envelope already set"`. This is reachable from external input paths when API and gossip race:

1. API handler receives envelope, sleeps for slot boundary alignment
2. Gossip envelope arrives during the sleep, gets added first
3. API handler wakes up and calls `addPayloadEnvelope()` → throws → 500 error to the VC

This is relevant in multi-node or DVT setups where the proposer's envelope arrives via gossip before the API call completes (similar to https://github.com/ChainSafe/lodestar/pull/8849).

## Proposed fix

Make `addPayloadEnvelope()` idempotent — silently return (or log) if the envelope is already set, matching the dedup semantics of `shouldImport()`. At minimum, the API handler should catch this gracefully instead of returning a 500.

## References

- PR #8962 — `payloadEnvelopeInput.ts` ([review comment](https://github.com/ChainSafe/lodestar/pull/8962#discussion_r2965822312))
- Issue #8915 — parallel block + payload submission
- PR #8849 — similar race condition handling

/cc @nflaig @twoeths

Start in payloadEnvelopeInput.ts at PayloadEnvelopeInput.addPayloadEnvelope(), then read the API and gossip submission paths described in the issue and the related PR references. Verify that a second envelope from the race is handled without an exception or API 500, while existing deduplication behavior remains intact.

Written by the indexing model from the issue text.