Add exhaustive initialization lint to DelayedInboxMessage

[cats2101]

Summary

• Annotate DelayedInboxMessage with // lint:require-exhaustive-initialization
• Update all initialization sites across production code and tests to explicitly set all four fields (BlockHash, BeforeInboxAcc, Message, ParentChainBlockNumber)
• One struct per PR as recommended in Use the struct-initialization linter more #3121

Changes

Annotation added:

• arbnode/mel/messages.go — DelayedInboxMessage struct

Production code fix:

• arbnode/inbox_tracker.go — added missing BlockHash field

Test fixes (added missing zero-value fields):

• arbnode/delayed_seq_reorg_test.go — 9 init sites, added ParentChainBlockNumber
• arbnode/mel/extraction/delayed_message_lookup_test.go — 2 init sites
• arbnode/mel/extraction/message_extraction_function_test.go — 2 init sites
• arbnode/mel/extraction/messages_in_batch_test.go — 3 init sites (+ added common import)
• arbnode/mel/runner/backlog_test.go — 1 init site (file deleted upstream)
• arbnode/mel/runner/database_test.go — 2 init sites
• arbnode/mel/runner/mel_test.go — 1 init site
• mel-replay/merkle_tree_fetcher_test.go — 1 init site (file deleted upstream)
• mel-replay/delayed_message_db_test.go — 1 init site

Test plan

• go test ./linters/structinit/ passes
• make lint (requires full build environment with solgen)
• make test on affected packages

Closes #3121

[github-actions]

Thank you for your contribution. However, this PR has been automatically closed because it was labeled as trivial. As stated in our CONTRIBUTING.md:

---

Important

Please, do not send pull requests for trivial changes; these will be rejected.
These types of pull requests incur a cost to reviewers and do not provide much value to the project.
If you are unsure, please open an issue first to discuss the change.
Here are some examples of trivial PRs that will most-likely be rejected:

• Fixing typos
• AI-generated code
• Refactors that don't improve usability

---

We appreciate meaningful contributions!