Shell/mempool: indicate fee needed to replace old operation in Conflict_error

Context

Fixes #5197 (closed) by indicating, in the Operation_conflict error, the fee that the operation would need to replace the old conflicting operation.

Depends on !8699 (merged) and !8842 (merged).

Manually testing the MR

You can try and inject conflicting operations into a sandbox node.

The fee indicated in the error is tested by:

dune exec tezt/tests/main.exe -- -f prevalidator.ml -t "Alpha: Manager_restriction_inject_isolated_node"

Additional tests involving non-manager operations are planned as a follow-up: see #5541 (closed).

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• n/a Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR