docs: correct the default agent model to spark-1-pro (#4371)

The API applies spark-1-pro when a request to POST /v2/agent does not
set model. Some SDK comments named spark-2 as the default. The README
table named spark-1-mini as the default. Both statements are wrong.

This change corrects the default in the README table, the Python SDK
docstrings, the Rust field comment, the Elixir option doc, the Java
builder comment, and one Python test comment.

All three models stay selectable. Only the default claim changes. The
Rust SDK changelog keeps its original text, because it records the
state at that release.

Co-authored-by: Claude <noreply@anthropic.com>