feat: schedule process_subsidies call after initiate_epoch_change

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Description

Call process_subsidies after initiate_epoch_change in InitiateEpochChangeOperation::invoke to distribute usage-independent subsidies right after epoch transition, since the new contract usage independent subsidies only pay for previous epoch's subsidy.

The call runs regardless of whether initiate_epoch_change succeeds (since another node may have already initiated it), and failure is best-effort (logged but does not block the epoch change result)

Test plan

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Pull request overview

Schedules a best-effort process_subsidies call immediately after attempting initiate_epoch_change, so usage-independent subsidies for the just-finished epoch can be paid right after an epoch transition.

Changes:

• Add a post-epoch-change process_subsidies call in InitiateEpochChangeOperation::invoke, gated by whether the subsidies object is configured.
• Add unit and E2E coverage to validate the post-epoch-change subsidy processing behavior.
• Expose latest_subsidized_epoch() (test-utils only) to assert that usage-independent subsidies advanced.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
crates/walrus-sui/src/types/move_structs.rs	Adds a test-only accessor for the latest subsidized epoch from the deserialized WalrusSubsidies object.
crates/walrus-service/src/node/epoch_change_driver.rs	Calls process_subsidies after the epoch-change initiation attempt and adds a unit test verifying the sequencing.
crates/walrus-e2e-tests/tests/test_client.rs	Adds an E2E test asserting latest_epoch advances after an epoch change due to the new post-epoch-change call.

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[karlwuest]

LGTM (after the comments are addressed)!

[github-actions]

Warning: This PR touches the mainnet-contracts directory. This should only be done if the Mainnet contracts are updated.
cc @karlwuest @mlegner

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[karlwuest]

Thanks for all the changes, LGTM!