Branching a conversation with tool authority facts is rejected as an auth failure #2060

Summary

Creating a new conversation from context can fail after a completed turn contains durable tool execution facts. The Desktop toast reports an authentication failure even though provider credentials are not involved.

Reproduction

1. Run a Desktop conversation that records a tool dispatch/operation reference.
2. On the completed turn, choose the action to create a new conversation from that context.
3. Observe that no child conversation is created and the UI reports an authentication failure.

Actual result

The Runtime rejects branchFromTurn() with:

branch_runtime_fact_rewrite_unsupported
Conversation copy contains durable runtime authority facts that require typed identity rewriting

The generic error classifier then matches the auth substring inside authority and presents the failure as authentication-related.

Root cause

cloneConversationRuntimeLedger() already assigns new Run, invocation, RuntimeEvent, and tool operation identities and atomically rewrites tool dispatch, recovery, and refs.operationId references. A later continuation-safety preflight blocks all tool authority facts before that typed copy path can run.

Continuation claims are different: their claim/boundary evidence is outside the ordinary Run/Event copy boundary, so V1/V2 continuationSource and continuationStart must remain fail-closed until a typed authority-copy protocol exists.

Separately, generalizedErrorMessage*() uses includes('auth'), which classifies unrelated words such as authority as authentication failures.

Expected result

• Branch/revision copies containing tool dispatch, recovery, or operation references use the existing typed identity rewrite and succeed.
• Continuation authority remains rejected before target Session creation.
• Authentication classification matches explicit authentication terms or HTTP 401/403, not arbitrary auth substrings.
• Regression coverage exercises branch, branch-before, and revision paths and verifies the copied operation identity differs from the source.