Prevent partial VEA epoch details after indexer request failures

[coderabbitai]

Summary

Prevent fetchEpochDetail from returning partial VEA epoch data when an inbox message-page request or an outbox relay-message batch request fails.

Required changes

• In veashi-scanner/lib/vea/client.ts, distinguish failed gql requests from valid empty message pages and valid empty relay batches.
• Propagate an indexer failure from fetchEpochDetail when either request type fails. Do not return partial message or relay status data.
• In veashi-scanner/hooks/useVeaEpoch.ts, handle that failure as a load error.

Rationale

The current behavior can stop message pagination early after a failed inbox request. It can also omit relays after a failed outbox request. The detail view can then display an incorrect bridge state.

Affected areas

• veashi-scanner/lib/vea/client.ts
• veashi-scanner/hooks/useVeaEpoch.ts

Acceptance criteria

• A valid empty message page ends pagination normally.
• A failed inbox message-page request does not produce partial epoch detail data.
• A valid empty relay batch remains supported.
• A failed relay batch does not mark affected messages as pending or unexecuted.
• useVeaEpoch exposes the indexer failure as a load error.
• Tests cover failed and valid-empty responses for both request types.

Backlinks

• Pull request: Chore/migrate veascan #513
• Review comment: Chore/migrate veascan #513 (comment)
• Requested by: @mani99brar