types: preserve tool call function indexes

[er-s-an]

Ollama's native tool parser assigns an incrementing index to each tool-call function. Message.ToolCall.Function currently drops that field when parsing native chat responses.

Add index: Optional[int] = None so callers can read and preserve this server-provided metadata. Responses without an index still parse, and exclude-none serialization keeps their existing shape.

Seven regression cases cover zero and nonzero indexes, attribute and mapping access, response/history serialization, old payloads without an index, and both sync/async chat with streaming on and off.

Validation on Python 3.12.14:

• New regressions on unchanged main: 6 failed, 1 compatibility control passed.
• Full suite and doctests: pytest ollama tests -q -> 127 passed.
• Ruff lint/format checks and git diff --check passed.
• Wheel/source builds, lock consistency, and requirements export checks passed.

The transport tests use a local mock HTTP server. No live model inference or broader Python/OS matrix was run.

Prepared with AI assistance and independently reviewed by a second AI agent. No human pre-review is claimed.