linera-chain: fold execute_block inputs into a BlockExecution enum

[ndr-ds]

Motivation

Follow-up to #6471, addressing the review comment from afck: after #6471, ChainStateView::execute_block took replaying_oracle_responses, policy, and phase as three separate arguments, but their legal combinations are fully determined by the phase. The type therefore permitted illegal inputs, and one coupling was guarded at runtime by an assert!:

• oracle responses are None when staging or validating a proposal, Some when executing a confirmed certificate;
• the bundle-failure policy AutoRetry is only legal when staging a proposal (the block is still being built); validating or confirming must Abort.

Proposal

Merge the three arguments into one BlockExecution enum whose variants carry exactly the legal data:

• StageProposal { policy } — caller-chosen policy (may be AutoRetry); never replays.
• HandleProposal — always Abort; never replays.
• HandleConfirmed { oracle_responses } — always Abort; replays the recorded responses.

execute_block now takes a single BlockExecution and derives the oracle-responses/policy internally. Payoff:

• the illegal (AutoRetry, Some(oracle_responses)) combination is now unconstructable, so the assert!(replaying_oracle_responses.is_none()) in execute_block_inner is deleted;
• execute_block drops its #[expect(clippy::too_many_arguments)] (down to 5 args);
• the two abort call sites no longer pass committed() explicitly.

round is deliberately left a positional parameter: its None-for-confirmed invariant is exclusion-only (both proposal phases carry a round), so folding it in would duplicate the field across variants and delete no guard.

Pure refactor, no behavior change. The Copy BlockExecutionPhase metric label is kept and derived via BlockExecution::phase().

Test Plan

• cargo clippy -p linera-core -p linera-chain --features "metrics test" --all-targets — 0 warnings.
• cargo build -p linera-chain -p linera-core (default features, metrics off) — clean.
• cargo doc -p linera-chain with RUSTDOCFLAGS=-D warnings — clean (intra-doc links on the new enum).
• cargo test -p linera-chain --features "metrics test" --lib — 57 passed.
• cargo test -p linera-core --features "metrics test" --lib -- test_handle_block_proposal test_handle_certificate — 20 passed.
• nightly fmt clean.

Release Plan

• Nothing to do / These changes follow the usual release cycle. (Pure refactor; no protocol or storage format change.)

Links

• Follow-up to Label block-execution metrics by phase #6471.