fix(agent): forward AionRS provider token usage

[minhtq1234]

Summary

• forward AionRS provider input/output token totals on successful finish events
• convert cumulative engine counters into per-turn deltas, including resumed sessions
• preserve optional-field compatibility through the event sink and WebSocket relay

Why

WePrompt's local token ledger receives finish events, but AionRS previously discarded AgentResult.usage and emitted a finish event without counters. Real Kimi turns therefore left Today / Week / Month totals at zero.

Verification

• cargo fmt --all -- --check
• cargo clippy -p aionui-ai-agent -p aionui-conversation --all-targets -- -D warnings
• cargo test -p aionui-ai-agent -p aionui-conversation
• CARGO_PROFILE_DEV_DEBUG=0 CARGO_PROFILE_TEST_DEBUG=0 just push -u contributor codex/bug015-provider-usage-v0150 — 8,366 passed, 24 skipped
• live Kimi K3 smoke in WePrompt: provider counters reached the local ledger, appeared in the usage popover, and survived renderer reload

Compatibility

The new finish-event fields are optional and omitted when no provider counters advance. Existing ACP and older consumers remain compatible.