chore: removed custom pre_dispatch for pallet_midnight

[Klapeyron]

Overview

Non-functional refactor. pallet_midnight no longer overrides ValidateUnsigned::pre_dispatch; it uses the Substrate trait default, which forwards to validate_unsigned(TransactionSource::InBlock, call) and maps the result to Ok(()) / TransactionValidityError.

To keep behavior identical to the old custom pre_dispatch, validate_unsigned now branches on TransactionSource: the in-block path uses the real block context and LedgerApi::validate_guaranteed_execution (what the removed pre_dispatch did); the local/external path keeps the existing pool validation (LedgerApi::validate_transaction, adjusted tblock, tags, longevity, etc.). No change to what the runtime accepts or rejects—only where that logic lives so the default pre_dispatch is correct.

📌 Submission Checklist

• Changes are backward-compatible (or flagged if breaking)
• Pull request description explains why the change is needed
• Self-reviewed the diff
• I have included a change file, or skipped for this reason:
• If the changes introduce a new feature, I have bumped the node minor version
• Update documentation (if relevant)
• Updated AGENTS.md if build commands, architecture, or workflows changed
• No new todos introduced

🧪 Testing Evidence

Please describe any additional testing aside from CI:

• Additional tests are provided (if possible)

🔱 Fork Strategy

• Node Runtime Update
• Node Client Update
• Other:
• N/A

Links