fix: reconcile gateway with v5 protocol changes

[patrickleet]

Why

The gateway and protocol changes passed independently but exposed two integration failures when combined on v5 (PR #227).

Changes

• Give ProtocolPreparationError engine-scoped visibility, matching the protocol_accumulator method consumed by sibling read-routing code. It remains outside the public API.
• Add the compiler-emitted protocolHash to the unique-key runtime fixture. The byte-exact assertion remains unchanged.

No error classification, authorization checks, or tests are weakened. No user-facing copy changes.

Validation

• cargo test -p distributed_cli --lib: 250 passed.
• Gateway GraphQL, operation, causal transport, query protocol, identity, GraphQL SQLite and SQLite repository suites: 81 passed, 3 existing ignored tests, 0 failures, with gateway-graphql-native,gateway-delivery,sqlite enabled.
• git diff --check: passed.
• Rust quality CI run 34310936117: success, including native/kind browsers and live celld.
• Gateway CI run 34310935513 attempt 2: success, including production/dev auth flows and controlled restart/reloads.

Intermittent test observed

The initial gateway application run failed the delivery-enabled Todo DOM-continuity assertion during token refresh. Three local runs (including 6x CPU throttling) and the CI rerun passed without changing that test or runtime behavior. The underlying timing issue remains unexplained; it is not claimed fixed by this PR. Temporary diagnostics were removed. All other initial CI checks passed.

Follow-up to #228 and #236/#237. Targets v5 to repair #227.

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