feat(models): add Anthropic provider support

[galuszkm]

Description

Adds anthropic as a built-in model provider, enabling direct access to the Anthropic API (not via Bedrock).
Replaces match/case blocks in models.py and loaders.py with if chains for consistency.
Updates docs, README, and lockfile deps.

Related Issues

N/A

Type of Change

• New feature

YAML / API Impact

New provider: anthropic value is supported in the models config block. Backwards-compatible — no existing provider keys or config structure changed.

models:
  claude:
    provider: anthropic
    model_id: claude-sonnet-4-6
    params:
      max_tokens: 4096  # required by the Anthropic API

Requires ANTHROPIC_API_KEY env var or params.client_args.api_key.

Testing

How have you tested the change?

• I ran uv run just check (lint + type check)
• I ran uv run just test for overall testing
• I added or updated tests that prove my fix is effective or my feature works
• I verified existing examples in examples/ still work

Checklist

• I have read the CONTRIBUTING document
• I have updated the documentation accordingly
• My changes generate no new warnings
• Any dependent changes have been merged and published

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