Refresh OpenAI and Anthropic model catalogs

[iskakaushik]

The model constants predate the current provider lineups: the
GPT-5.6 family and GPT-5.5 were missing, as were Claude Fable 5,
Opus 5, Opus 4.8, and Sonnet 5, while long-retired identifiers
(GPT-4o, GPT-4, GPT-3.5, the o-series, Claude Sonnet/Opus 4.0) were
still advertised. Add the current identifiers with explicit dated
snapshot constants, remove the retired ones, and move the defaults
to gpt-5.6 and claude-sonnet-5. Removing the retired constants is a
source-compatibility change for callers still referencing them.

Two request-building accommodations come with the new models.
GPT-5.6 defaults to reasoning on Chat Completions, which does not
support function tools and can consume the whole completion budget
without producing user-visible text, so requests pin
reasoning_effort to "none". Recent Claude models reject sampling
controls, so temperature and top_p are omitted for those model IDs
with a warning log.

Update supported_models(), mocks, fixtures, unit and integration
tests, examples, and documentation accordingly. Tests that need a
neutral model now use the literal "test-model" instead of a real
identifier.

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