Extend ChatCompletion with Claude 4.x capabilities

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Part of #75.

## What

Extend the existing `ChatCompletion` task with all missing Claude 4.x / Anthropic API properties. These are additive changes — no breaking changes to existing flows.

## Checklist

### New input properties

- [ ] **Extended thinking** — `thinking` (`Boolean`, default `false`) + `budgetTokens` (`Integer`). Maps to `ThinkingConfigEnabledParam`. Enforce `temperature = 1` when enabled.
- [ ] **Vision / image input** — allow `ChatMessage.content` to carry image content blocks (URL, base64, `file_id`) in addition to plain `String`. Maps to `ImageBlockParam`.
- [ ] **PDF / document input** — document content blocks (URL, base64, `file_id`) in `ChatMessage.content`. Maps to `DocumentBlockParam`.
- [ ] **`toolChoice`** — enum `AUTO` | `ANY` | `TOOL` (+ `toolChoiceName`) | `NONE`. Maps to `ToolChoiceAutoParam` etc.
- [ ] **`stopSequences`** — `List`. Passed to `MessageCreateParamsBuilder.stopSequences(...)`.
- [ ] **Granular `cacheControl`** — optional `cacheControl: ephemeral` field on `SystemPromptBlock`, `ChatMessage`, and `Tool` for per-block cache control (instead of / in addition to the global `promptCaching` flag).
- [ ] **`stream`** — `Boolean` (default `false`). When true, use `createStreaming(...)`, log text deltas incrementally, and write the full text to `Output.outputText` at the end.
- [ ] **`metadata.userId`** + **`serviceTier`** — optional passthrough for enterprise cost attribution and priority tier.
- [ ] **`betas`** — `List` to pass `anthropic-beta` headers without a plugin release.
- [ ] **Client configuration** — `baseUrl`, `connectTimeout`, `readTimeout`, `maxRetries` on `AbstractAnthropic`, wired into `AnthropicOkHttpClient.builder()`.

### New output fields

- [ ] **`thinking`** — reasoning text blocks (when extended thinking is on).
- [ ] **`citations`** — list of `Citation` records from document responses.
- [ ] **`stopSequence`** — the matched stop string (alongside existing `stopReason`).

### Bug fix

- [ ] Tool-input JSON conversion failure in `ChatCompletion.run` (~line 340) silently leaves `input` null — log a warning instead.

## Acceptance

Each bullet above can land in the same PR or be split across focused PRs. Tests and a docs example required per property.

Locate the existing ChatCompletion task and AbstractAnthropic client configuration, then review their current request, response, streaming, and error-handling paths. Split the checklist into focused changes, adding tests and a documentation example for each property; done means all requested Claude 4.x inputs, outputs, client options, and the tool-input warning are covered without breaking existing flows.

Written by the indexing model from the issue text.