Support anthropic streaming tool calls

[knjiang]

This PR adds support for anthropic streaming tool calls.

[knjiang]

this is the big one to remove choices[*].delta.tool_calls and what was blocking loop anthropic from working.

[knjiang]

the way the testing works here is the following:
we have coverage-report binary that produces an HTML matrix report -> https://github.com/braintrustdata/lingua/actions/runs/21727047150?pr=81

^ the HTML report is helpful for when u want the AI to crank at supporting smth.

in addition we run https://github.com/braintrustdata/lingua/blob/main/crates/coverage-report/tests/cross_provider_test.rs

which runs cargo test on the coverage-report matrix and basically it'll fail if we see an "unexpected diff" where it's not listed in crates/coverage-report/src/streaming_expected_differences.json.

curerntly we don't have full support for all providers so we are flagging them in one by one, the "strict" mode tests only run for:

const REQUIRED_PROVIDERS: &[ProviderFormat] = &[
    ProviderFormat::Responses,
    ProviderFormat::OpenAI, // ChatCompletions
    ProviderFormat::Anthropic,
];