Track Claude 4.x / Anthropic API feature gaps: extended thinking, vision, server-side tools, batch, files, streaming

## Background

Plugin currently exposes a single `ChatCompletion` task on top of `com.anthropic:anthropic-java:2.27.0`. Many Claude 4.x / Anthropic API capabilities are not yet wrapped. This issue tracks the gaps so they can be split into focused PRs.

## Out of scope (for now)

- Bedrock / Vertex backend variants (track separately if there is demand).
- Streaming as a dedicated task (folded into `ChatCompletion.stream` in #90).

## Acceptance

Each sub-issue lands as its own PR with tests and a docs example. #90 is the highest priority as it unblocks the most common user requests (thinking, vision, streaming).

Start by reviewing the existing ChatCompletion task and the Anthropic Java dependency, then use the listed gaps to define a focused sub-issue; streaming is already tracked in #90. Done means one focused capability lands in its own PR with tests and a documentation example, as required by the acceptance criteria.

Written by the indexing model from the issue text.