[Fix] - Add sanity check for final forced tx number

[thedarkjester]

This PR implements issue(s) #

Checklist

• I wrote new tests for my new core changes.
• I have successfully ran tests, style checker and build against my new changes locally.
• I have informed the team of any breaking changes if there are any.

---

Note

Low Risk
Small, additive validation that only introduces an earlier revert in finalization when forced-transaction state is inconsistent; covered by a new test.

Overview
Adds a sanity check during finalizeBlocks to ensure that when finalForcedTransactionNumber is non-zero, the corresponding forcedTransactionRollingHashes entry is present; otherwise it reverts with new custom error MissingRollingHashForForcedTransactionNumber.

Updates the forced-transactions interface to expose this new error and adds a Hardhat test covering the new revert path when finalization references a forced transaction number without a stored rolling hash.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.70%. Comparing base (2fc6ea9) to head (69b4997).

Additional details and impacted files

@@                Coverage Diff                 @@
##             feat/tx-gateway    #2379   +/-   ##
==================================================
  Coverage              61.70%   61.70%           
  Complexity              1618     1618           
==================================================
  Files                    440      440           
  Lines                  17356    17358    +2     
  Branches                1945     1946    +1     
==================================================
+ Hits                   10709    10711    +2     
  Misses                  5995     5995           
  Partials                 652      652           

Flag	Coverage Δ		*Carryforward flag
hardhat	98.00% <100.00%> (+<0.01%)	⬆️
kotlin	57.08% <ø> (ø)		Carriedforward from 2fc6ea9

*This pull request uses carry forward flags. Click here to find out more.

Files with missing lines	Coverage Δ
contracts/src/rollup/LineaRollupBase.sol	100.00% <100.00%> (ø)

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