Move blob fee tracking into execute_operation.

[afck]

Motivation

Ideally fees should be tracked as closely as possible in the code to where the operation that caused them is processed. Currently we are tracking fees for publishing blobs all at once at the start of execute_block. (See discussion here: https://github.com/linera-io/linera-protocol/pull/3546#pullrequestreview-2698125145)

Proposal

Move that logic into execute_operation.

Test Plan

Only refactoring.

Release Plan

• Nothing to do / These changes follow the usual release cycle.

Links

• Original PR: https://github.com/linera-io/linera-protocol/pull/3546
• reviewer checklist