refactor(core): use nonempty array guards

[kitlangton]

Why
Three private paths manually prove non-emptiness by casting or reconstructing first-plus-rest tuples. Effect Array already supplies guards and mapping overloads that preserve the non-empty shape.

What Changes
Uses isReadonlyArrayNonEmpty for provider-hosted tool content, isArrayNonEmpty for normalized local tool results and flattened child-process pipelines, and Effect Array map for non-empty tool-content redaction. The already-normalized arrays are returned or published directly.

Scope
Hosted empty content still falls back to text, local empty content still dies, cross-spawn still throws on an impossible empty flatten result, pipeline command/options ordering is unchanged, and every transferred content item uses the same sanitizer. PRs #45336 and #44806 touch other publisher state, while #45407 touches cross-spawn exit handling; none implement these collection refinements.

Verification

cd packages/core
bun typecheck
bun run test test/session-runner-tool-events.test.ts test/session-create.test.ts test/effect/cross-spawn-spawner.test.ts
cd ../util
bun typecheck
cd ../..
bunx oxlint packages/core/src/session/runner/publish-llm-event.ts packages/core/src/session/transfer.ts packages/util/src/cross-spawn-spawner.ts

Core and Util typechecks passed, and all 84 focused tests passed. Oxlint completed with no errors and 36 pre-existing warnings. The push hook completed the 33-package workspace typecheck.