fix(mcp-apps): Lazily discover UI tools for cloud runs

[charlesvien]

Problem

MCP app cards (like the loop builder's review card) only render if the desktop has started a local agent session since app launch. That's because the app only learns which MCP tools have a UI during a discovery step that a local session triggers. Cloud runs (loop builder, repo-less chat) never start a local session, so the check always says "no UI" and the card silently never appears. The agent then points users at a Create button that doesn't exist.

The same gap means cloud runs fetch app HTML without its CSP metadata, so since #3803 they get the restrictive default CSP.

Changes

When a UI lookup misses and the server hasn't been discovered yet, connect and discover it on the spot instead of waiting for a local session. Resource fetches also load CSP metadata first. A failed discovery throws with a 60s backoff so the UI retries later instead of caching "no card" forever.

How did you test this?

• 7 new unit tests in @posthog/core covering the lazy path, failure backoff and CSP attachment.
• biome lint packages/core and typecheck on core and host-router, all clean.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

[trunk-io]

😎 This pull request was merged.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 68b361f.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "discover MCP UI tools lazily for cloud r..." | Re-trigger Greptile}

[github-actions]

Contained core-package bugfix (lazy MCP UI discovery for cloud runs) with matching new unit tests covering the added logic paths; no auth/billing/CI/dependency/migration surface touched, and a Greptile bot review with a positive reaction adds further signal.

• 👍 on the PR from greptile-apps[bot].

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	196L, 1F substantive, 317L/2F incl. docs/generated/snapshots — within ceiling
tier	✓	T1-agent / T1d-complex (317L, 2F, single-area, fix)
stamphog 2.0.0b3		.stamphog/policy.yml @ 05b4aa0 · reviewed head ac9e493

New commits pushed (delta classified non_trivial_delta) — stamphog approval dismissed; re-review running automatically.

[stamphog]

Note

🤖 stamphog reviewed 68b361fa539e688be390bc1cb94ae44680975fd7 — verdict: ESCALATE

This changes how CSP metadata is warmed and attached to MCP app UI resource fetches — a security-sensitive surface controlling what embedded HTML can do — and despite solid test coverage, it has no independent review (0 top-level reviews; only a Greptile discussion comment plus a lone 👍, and no owning-team or strong-familiarity author signal).

• 👍 on the PR from greptile-apps[bot].
• Touches CSP metadata caching/attachment for MCP app UI resources (security-relevant surface) without a formal review (0 top-level reviews).
• No ownership match and author familiarity with these lines isn't reported as STRONG, so there's no independent-assurance path other than a human review.
• The only external signal is a Greptile discussion comment + 👍 reaction, which is weak evidence, not a substantive review with no unresolved concerns.

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	243L, 1F substantive, 472L/2F incl. docs/generated/snapshots — within ceiling
tier	✓	T1-agent / T1d-complex (472L, 2F, single-area, fix)
stamphog 2.0.0b3		.stamphog/policy.yml @ 05b4aa0 · reviewed head 68b361f