Add real Excel adversarial extension provider coverage

[tmustier]

Summary

• add a repeatable real-Excel adversarial smoke for pasted-code sandbox model providers
• drive permission grants, host connection setup, dynamic discovery, model selection, streaming, endpoint upgrades, unload races and hostile endpoint registration through the tokened taskpane bridge
• keep test credentials inside instrumented local HTTPS gateways and return only bounded pass/fail summaries

Defects the smoke found and fixes in this PR

1. Sandbox bootstrap did not execute in Excel — an unescaped newline made the generated module syntactically invalid; generated JavaScript now has a parser regression test.
2. Discovery was unbounded — responses are now limited to 2 MiB, 2,000 entries and 256 characters per model ID while retaining the last safe cache on rejection.
3. Same-ID endpoint upgrades could retain stale session metadata — provider refreshes now reconcile runtime metadata, and reselection updates transport metadata even when provider/model identity is unchanged.
4. In-flight unload could leave a removed provider selected — model reconciliation retries after agent_end once the action queue unwinds.
5. Delayed discovery could resurrect cache after unregister — dynamic providers now own lifecycle abort controllers and cancel discovery before cache deletion.

Real Excel evidence

npm run smoke:extension-provider passed against a pasted inline extension running in sandbox-iframe mode with two instrumented HTTPS gateways:

• default capability denial followed by explicit grants
• host-owned discovery/inference credentials; sandbox secret read denied
• exact EXTENSION_PROVIDER_A_OK, EXTENSION_PROVIDER_CACHE_OK, EXTENSION_PROVIDER_B_OK, and EXTENSION_PROVIDER_DELAYED_OK
• offline cache fallback and oversized catalogue rejection
• gateway A → B cache rebinding with no replacement credential sent to A
• in-flight unload, idle uninstall and delayed-discovery race recovery
• non-allowlisted endpoint host rejected

Evidence: docs/release-smoke-runs/2026-07-16-macos-extension-provider.md

Automated verification

• npm run check
• npm test — 937 passing (60 model, 745 context, 108 security, 16 manifest)
• npm run build
• npm run validate
• npm audit --audit-level=high — 0 high/critical; existing inherited moderate findings only

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
pi-for-excel	Ignored		Jul 16, 2026 5:50pm