Transaction preparation can panic in committor workers

@snawaz is already working on this.

Since Sep 9, 2026.

## Problem

**Report**
What happened:
In `magicblock-committor-service/src/transaction_preparator/mod.rs` at line 106, there is a `.expect()` call that can cause a runtime panic in committor workers:
```rust
let message = TransactionUtils::assemble_tasks_tx(
authority,
&tx_strategy.optimized_tasks,
self.compute_budget_config.compute_unit_price,
&lookup_tables,
)
.expect("Possibility to assemble checked above")
.message;
```

Expected:
Error should be propagated via `?` through the existing `PreparatorResult` error path instead of panicking.

Repro:
The lookup tables passed to the second `assemble_tasks_tx` call may differ from the first check at line 86, making the assumption in `.expect()` not strictly guaranteed.

Logs/links:
https://github.com/magicblock-labs/magicblock-validator/blob/master/magicblock-committor-service/src/transaction_preparator/mod.rs#L106

I'd like to work on this fix if no one is assigned yet.

**Commit / version**
master

**Environment**
_No response_

## Reproduction

Not yet documented.

## Expected

The affected flow completes without the described failure.

## Context

Add the affected revision and path when known.