Fix conversation fork workspace context

[Paul Lizer (paullizer)]

Summary

• prevent fork conflict logging from converting expected responses into HTTP 500
• support owned group/public-grounded single-user conversations after workspace access revalidation
• preserve authorized context and normalized chat type in the fork
• align browser eligibility and add backend, route, and UI regression coverage
• document the fix and release it as version 0.250.101

Validation

• 14 fork backend tests passed
• 9 conversation route tests passed
• route policy inventory, unauthenticated contract, and policy coverage tests passed
• added-line XSS and broken-access-control guardrails passed

Closes #1025