[plugins] Anthropic Python: refresh model registry (add sonnet-4-6, drop retired 3-5-*)

[hilariie]

Parent: #5617 (Phase A, P0: item 18)

Why

The Python static model registry has drifted from both the JS reference set and the current Anthropic lineup. It is missing claude-sonnet-4-6 (which the JS KNOWN_MODELS set advertises) and still advertises the retired claude-3-5-haiku and claude-3-5-sonnet. Advertising retired models while omitting a current one is misleading in reflection and the dev UI, and breaks parity with JS.

This is a low-risk parity change: it is isolated to the model-info module, has no upstream or downstream dependencies, and nothing else in the epic blocks on it, so it can land early as a standalone change.

What needs to be done

• Bring the default known-model set in line with the JS static set and the current API: surface claude-sonnet-4-6, stop advertising the retired claude-3-5-* models, and keep the existing 4.x entries.
• Keep limited-access and testing-blocked models out of the default static set while remaining reachable:
  • claude-fable-5: present in the JS set but deferred here due to testing limitations.
  • claude-mythos-5: access-gated and absent from JS KNOWN_MODELS.
    Both must stay usable via generic model references and via dynamic listing (item 19, [plugins] Anthropic Python: dynamic model listing via beta.models.list with static fallback #5633 ), just not in the default set.

Acceptance

• claude-sonnet-4-6 is advertised; retired claude-3-5-* entries are no longer advertised; existing 4.x entries are retained.
• claude-fable-5 and claude-mythos-5 are absent from the static set but still resolve when referenced by full name.
• Model/registry tests updated; CI green.

Reference

• Current Python set: py/plugins/anthropic/src/genkit/plugins/anthropic/model_info.py:166-178; generic fallback get_model_info() at model_info.py:189-204.
• JS target: js/plugins/anthropic/src/models.ts:105-117 (KNOWN_MODELS).