feat(anthropic): Claude 5.x safety defaults (prefix error, temporary fallback block, Mythos 5 on Converse) #4652

Goal

Refuse Claude 5.x configurations that docker-agent cannot serve correctly yet, instead of sending them:

1. Non-empty fallbacks is rejected when any model in the chain is a 5.x model, until feat(Claude): end-to-end server fallback for Claude 5.x #4584 lands.
2. Mythos 5 on Bedrock Converse is refused before any inference call.

Scope change (2026-10-20): making thinking_prefix_mismatch: error the 5.x default moved to #4653. The runtime routinely edits the conversation prefix (num_history_items trimming, hooks, compaction), so error would make those sessions fail until #4653 can replay signed history exactly or refuse cleanly. Today's drop_block default stays, and dropped blocks are already logged.

Why

• fallbacks.go forwards any list with the June beta header and no checks (fallbacks.go). Server fallback changes model mid-response and sticks to the fallback for about an hour, which docker-agent doesn't handle yet (feat(Claude): end-to-end server fallback for Claude 5.x #4584).
• Mythos 5 is only available on Bedrock Mantle, not runtime or Converse (model card). Today the request fails at AWS with an unclear error.

Scope

• Fallback block: reject when the primary or any explicit target is one of the nine 5.x models, including a legacy primary with a 5.x target. Absent or empty fallbacks works as usual. Fallback chains with no 5.x model are unchanged. The error message points to feat(Claude): end-to-end server fallback for Claude 5.x #4584.
• Refuse Mythos 5 on Converse at client construction when the model ID makes it clear.

Acceptance criteria

• Fallback tests: 5.x primary, legacy primary → 5.x target, absent/empty list, a chain with no 5.x model (unchanged, control test).
• Mythos 5 on Converse fails before any network call; Mythos 5.1 on Converse is allowed.
• Docs and latest schema descriptions updated.
• task build, task test, task lint pass.

Parent: #4583