fix(plugins): deliver onStreamComplete to disk-installed plugins (#11825)

[pacocartones]

Summary

• onStreamComplete (shipped in v3.8.50, feat(plugins): add onStreamComplete built-in event exposing streaming usage and timing (#9571) #9669) was emitted on the gateway side but never delivered to disk-installed plugins — the plugin-facing wiring was missing, so no plugin could subscribe to it.
• Root cause, three layers: the manifest HooksSchema had no onStreamComplete field (Zod silently stripped it from plugin.json); loader.ts built no IPC wrapper for it; and manager.ts's hardcoded hookNames list never registered it. The event fired into an empty registry and was dropped. The existing chatcore-plugin-onresponse tests only passed because they call registerHook("onStreamComplete", …) directly — a path real plugins don't have.
• Fix: declare onStreamComplete as a manifest hook, wire it through the loader as a fire-and-forget notification (same pattern as the lifecycle hooks), and register it in the manager. The payload now also carries requestId (the request traceId) so consumers can correlate the stream-completion event with onRequest/onResponse for the same request.

Related Issues

• Closes [BUG] onStreamComplete is emitted internally but never delivered to plugins (missing wiring in manifest schema / loader / manager) #11825
• Related to feat(plugins): add onStreamComplete built-in event exposing streaming usage and timing (#9571) #9669

Validation

• Change type: plugins (manifest/loader/manager) + streaming finalization wiring
• Focused test: tests/unit/plugins-onstreamcomplete-delivery-11825.test.ts — drives the real install → activate → emit path (not registerHook directly). Verified failing on the pre-fix tree (manifest.hooks.onStreamComplete must survive validation; installed plugin should declare onStreamComplete; got "[]") and passing after (2/2).
• npm run lint (pre-commit prettier + eslint ran clean on all touched files)
• Reconciled with the current main; focused test rerun afterward
• Production-code changes include a new automated test in this PR

Tests Added Or Updated

• Added tests/unit/plugins-onstreamcomplete-delivery-11825.test.ts: installs + activates a plugin declaring onStreamComplete, emits through runPluginOnStreamCompleteHook, and asserts the plugin process receives the payload including requestId. Also asserts the manifest schema preserves the hook flag.

Reviewer Notes

• onStreamComplete is wired alongside the lifecycle hooks because it is a one-way fire-and-forget notification (no return value consumed); no blocking-hook path changes.
• No change to the unrelated internal onStreamComplete stream-finalization callback in streamFailureFinalization.ts / streamingCost.ts — that is a different concept.

[pacocartones]

CI note — the red checks are not this change's doing (delivering onStreamComplete to disk-installed plugins). Build was cancelled by fail-fast, not a compile error. Vitest (MCP / autoCombo / UI components) and Protocol Clients E2E (advisory) fail environmentally (green on main; the run logs ECONNREFUSED and a better-sqlite3 binding fallback), on tests unrelated to plugin delivery. Rebased onto current main.