feat(models): refresh OpenAI / Anthropic model identifiers (May 2026)

[iskakaushik]

Summary

Brings the public model identifier surface up to date for both providers and aligns the runtime `supported_models()` with the headers.

OpenAI (`include/ai/openai.h`, `src/providers/openai/openai_client.cpp`)

• Promote the GPT-5 family to current: `gpt-5.4` / `-pro` / `-mini` / `-nano`, `gpt-5-mini`, `gpt-5-nano`, plus `gpt-4.1` / `-mini`.
• Default model → `kGpt54`.
• Older identifiers (gpt-5, gpt-5.1, gpt-5.2, gpt-4o*, gpt-4-turbo, gpt-4, gpt-3.5-turbo, o1/o3/o4 variants) retained but marked DEPRECATED for backward compatibility.
• Drop unverified `gpt-5.5` / `gpt-5.5-pro` — neither appears in OpenAI's public catalog nor in the Vercel AI SDK's curated `OpenAIChatModelId` / `OpenAIResponsesModelId` unions.

Anthropic (`include/ai/anthropic.h`, `src/providers/anthropic/anthropic_client.cpp`)

• Promote `claude-opus-4-7`, `claude-sonnet-4-6`, `claude-opus-4-6`, `claude-haiku-4-5`. Default → `kClaudeSonnet46`.
• `supported_models()` now contains both the friendly aliases (`claude-sonnet-4-5`) and dated snapshots (`claude-sonnet-4-5-20250929`) so `client.supports_model(ai::anthropic::models::kClaudeSonnet45)` no longer returns false.

Tests / examples / mocks / README updated to the new defaults. Fixes the previously-failing `AnthropicIntegrationTest.DefaultModelGeneration` which still asserted on the old `claude-sonnet-4-5` default — assertion now derives from `kDefaultModel`.

Test plan

• `ctest -R OpenAIClient` — 13/13
• `ctest -R AnthropicClient` — 13/13
• `AnthropicIntegrationTest.DefaultModelGeneration` + `BasicTextGeneration` — 2/2 pass against real Anthropic API
• Examples build (basic_chat, multi_provider, tool_calling_*, etc.)