Fix resumed step message projection

[CooperSheroy]

Summary

• Preserve completed assistant text and reasoning parts when the runtime resumes a turn with a reused step index, such as after OAuth authorization.
• Keep active streaming text/reasoning updates in place while appending a new part once the previous part is done.
• Add a regression test for the OAuth resume event shape reported in Default message reducer overwrites text when OAuth resumes in same step #436.

Closes #436.

Validation

• corepack pnpm exec oxfmt packages/eve/src/client/message-reducer.ts packages/eve/src/client/message-reducer.test.ts .changeset/resumed-step-message-parts.md
• corepack pnpm exec oxlint packages/eve/src/client/message-reducer.ts packages/eve/src/client/message-reducer.test.ts
• corepack pnpm --filter eve exec vitest run --config vitest.unit.config.ts src/client/message-reducer.test.ts
• pnpm --filter eve run typecheck
• git diff --check

Note: local Node is v22.22.2 while the repo declares Node >=24, so pnpm emitted engine warnings during validation.

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