Add regression e2e for surfaced OAuth setup failure causes

[RhysSullivan]

Partially addresses #1330.

OAuth setup failures (oauth.probe / oauth.start against a server with broken discovery metadata) used to reach the sandbox only as Internal tool error [id], with the real cause visible solely in the daemon log. That was fixed by the user-actionable error contract (#1353); this adds the missing end-to-end guard for the exact setup-failure path.

Two cross-target scenarios, both driving the agent-facing tools through the MCP execute sandbox against a broken metadata server:

• oauth.probe fails with oauth_probe_error and a message naming the malformed metadata.
• oauth.start scope discovery fails with oauth_start_error carrying the Failed to discover OAuth scopes: … cause chain.

Both assert the opaque Internal tool error mask is not used.

Verification: green on the selfhost and cloud projects. Red-proof: temporarily disabling the user-actionable branch in the tool invoker makes both scenarios fail with Error: Error: Internal tool error [hex] — the original symptom.

No changeset: e2e-only, no published package changes.

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Torn down — the PR is closed.

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commit: 12ac1f4