fix: expose exact outbound session routes

[scotthuang]

Summary

• expose an exact outbound session route for canonical direct Weixin recipients
• resolve the real outbound account before constructing the session key
• reject non-direct target prefixes instead of claiming an exact route

This is the plugin-side companion to openclaw/openclaw#130871. That core change projects delivered heartbeat results only when a channel plugin certifies that the outbound recipient maps exactly to the canonical inbound session. The core also downgrades this certification when a binding belongs to another agent.

Validation

• npm run typecheck
• npm run build
• npx vitest run src/channel.test.ts — 3/3 passed
• full npm test — all 400 tests passed; the command exits non-zero only because the repository-wide pre-existing branch-coverage result is 89.15% against a 90% threshold. Excluding the new test still reports 89.13%, so this patch does not introduce the baseline gap.

No merge is requested until the corresponding generic OpenClaw host contract is accepted.