fix(mcp-store): fix MCP connect button loading state

[cvolzer3]

Problem

MCP server connection failures could leave the Connect button stuck in a loading state. Reconnecting an OAuth MCP server also hit the browser redirect boundary: the MCP Store authorize endpoint redirects to the provider, but browser fetch cannot expose a cross-origin Location header, which produced the status 0 failure.

Reconnect can also be blocked by a transient PostHog token-refresh 500 if the app preemptively refreshes a still-valid in-memory access token.

Changes

• Reset MCP template connection loading from the install mutation state so failed installs return the button to Connect.
• Expect MCP authorize to return a JSON redirect_url for PostHog Code instead of trying to inspect a manual browser redirect.
• Let core auth use the current in-memory access token when a preemptive refresh fails but the token has not actually expired.
• Preserve the existing logout behavior when the refresh token is rejected.
• Added regressions for failed install loading state, authorize JSON redirects, transient refresh failures, and refresh-token auth failures.

Backend companion PR: PostHog/posthog#65917

How did you test this?

I did not do a manual app walkthrough. I ran:

• pnpm --dir packages/ui exec vitest run src/features/mcp-servers/hooks/useMcpServers.test.tsx
• pnpm --filter @posthog/api-client test -- src/fetcher.test.ts src/posthog-client.test.ts
• pnpm --filter @posthog/api-client typecheck
• pnpm --filter @posthog/ui typecheck
• pnpm --filter @posthog/core test -- src/auth/auth.test.ts
• pnpm --filter @posthog/core typecheck
• pnpm exec biome lint packages/core
• pnpm exec biome check --write packages/ui/src/features/mcp-servers/hooks/useMcpServers.ts packages/ui/src/features/mcp-servers/hooks/useMcpServers.test.tsx
• pnpm exec biome check --write packages/api-client/src/fetcher.ts packages/api-client/src/posthog-client.ts packages/api-client/src/fetcher.test.ts packages/api-client/src/posthog-client.test.ts
• git diff --check

The pre-commit hook also ran pnpm typecheck before the latest Code commits.

Automatic notifications

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

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 54ea3d7.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(ui): reset MCP connect loading state" | Re-trigger Greptile}