Anthropic model constants are missing the Claude 5 family

`crates/rig-core/src/providers/anthropic/completion.rs` stops at `claude-opus-4-8` and `claude-sonnet-4-6`. Anthropic's current lineup is Claude Fable 5.1, Opus 5 and Sonnet 5:
https://platform.claude.com/docs/en/about-claude/models/overview

Same update as #1616, and #1370 before that. These go stale every time Anthropic ships, so it keeps coming back around.

Two other places need it or the constants are only half useful:

- `default_max_tokens_for_model` has no arm for the new IDs, so they fall through to the 2,048 fallback. All three are 128K.
- `supports_mid_conversation_system_messages` only checks Opus 4.8. Opus 5 and Fable support it, Sonnet 5 does not.

I have this working locally against main and can open a PR.

Start in crates/rig-core/src/providers/anthropic/completion.rs and inspect the model constants, default_max_tokens_for_model, and supports_mid_conversation_system_messages. Add the Claude Fable 5.1, Opus 5, and Sonnet 5 IDs with the stated token limits and system-message support, then verify the existing Anthropic provider tests or checks pass.

Written by the indexing model from the issue text.