[jihtsan]

Closes #1103.

Seed streaming usage from message_start, then apply each message_delta's non-nil cumulative fields in order. This preserves input/cache counts, explicit zero updates and untouched nested metadata without adding cumulative output counters. Keep the latest provided stop reason while retaining ordinary text/tool/thinking reconstruction and nonstream behavior. Add a changelog entry.

Validation on Ruby4.0.9/macOS arm64 with real Anthropic1.82.0 and offline fixture transport:

• Original provider/response baseline:27 examples passed. New corrected regressions on base:31 examples,6 failures.
• Final provider, response, Anthropic adapter/message and full Assistant specs:149 examples,0 failures,1 existing pending Ollama case. Repository fixtures verify text5+10=15, thinking12+30=42 and tools472+89=561, plus cache/zero/nil/later cumulative updates.
• Repository-wide bundle exec standardrb, bundle exec yardoc and git diff --check:passed (existing YARD warnings).
• Independent Standards and Spec reviews on the submitted SHA.

No tracked Gemfile.lock exists upstream; validation reused the retained generated lock with BUNDLE_FROZEN=true, outside this commit. Full Postgres/vector/provider integration and other Ruby/OS matrices were not run. No live model/API call was needed; hosted CI is not claimed passed.