fix: add SMT root mismatch verification in operator sender

[gitteri]

Summary

• Adds a critical safety check: before the operator proceeds with a withdrawal batch, it now verifies that the locally-computed SMT root matches the on-chain root stored in the instance account.
• If a mismatch is detected (e.g. after an operator crash between on-chain tx confirmation and local DB update), it logs detailed diagnostic info and returns a new SmtRootMismatch error instead of silently diverging.

Changed files

• indexer/src/error/transaction.rs -- new SmtRootMismatch error variant
• indexer/src/operator/sender/state.rs -- root verification logic before withdrawal processing

Test plan

• Verify operator starts normally when local and on-chain roots match
• Simulate a root mismatch and confirm the operator halts with the SmtRootMismatch error and diagnostic logs

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