feat(claude): add adaptive thinking + output effort for Opus 4.7+

[NicolasWalter]

What type of PR is this?

feat

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• The description is user-oriented and clear enough for others to understand.
• User documentation PR attached (not needed — the new API is documented in the adapter's own README / README_zh, both updated in this PR).

(Optional) Translate the PR title into Chinese.

feat(claude): 为 Opus 4.7+ 模型支持自适应思考（adaptive thinking）及输出精力等级（output effort）

(Optional) More detailed description for this PR

en:

Claude Opus 4.7 rejects the legacy {"type":"enabled","budget_tokens":N} thinking shape and requires {"type":"adaptive"} with an optional output_config.effort. The current adapter only emits the legacy shape, so calling claude.WithThinking(...) against Opus 4.7 returns:

invalid_request_error: "thinking.type.enabled" is not supported for this model.
Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior.

This PR adds an opt-in adaptive path. Changes are additive and backward-compatible — every existing caller keeps its current behavior because the empty Mode defaults to ThinkingModeEnabled.

Changes

• Thinking.Mode (ThinkingModeEnabled default, ThinkingModeAdaptive for Opus 4.7+). Empty mode preserves the pre-existing behavior.
• Thinking.Effort ("low" | "medium" | "high" | "max") → populates params.OutputConfig.Effort using the SDK's typed enum. Trim + case-insensitive; unknown values are silently ignored to match the rest of the adapter's leniency.
• Dispatch in genMessageNewParams: emit OfAdaptive for adaptive mode, OfEnabled (with BudgetTokens) otherwise. Effort applies to both.
• normalizeOutputEffort helper at the bottom of claude.go (next to getEnvWithFallbacks). utils.go stays untouched (no new imports there).
• README + README_zh snippet added next to the existing WithThinking example.
• TestThinkingConfig with 7 subtests covering the relevant paths: legacy default, empty mode default, adaptive with/without Effort, Effort normalization (trim + case), unknown Effort ignored, Enable=false short-circuit. All pass under mockey with the same style as the existing tests.

No new module dependency: the adapter already imports github.com/anthropics/anthropic-sdk-go (5 packages from it, actually) and the SDK exposes ThinkingConfigAdaptiveParam and OutputConfigParam since v1.25+. Nothing added to go.mod.

zh(optional):
本 PR 为 Claude 适配器增加 adaptive thinking 和 output effort 支持，以兼容 Opus 4.7 及更新模型。向后兼容：Mode 为空时保留原有 OfEnabled + budget_tokens 行为。新增 Thinking.Mode、Thinking.Effort 字段和 ThinkingModeEnabled / ThinkingModeAdaptive 常量；同时更新 README 示例，新增 7 个单元测试覆盖主要路径；未引入新的模块依赖（anthropic-sdk-go 已是适配器的直接依赖）。

(Optional) Which issue(s) this PR fixes

Fixes #780

(optional) The PR that updates user documentation

N/A — adapter-local README + README_zh updated in this PR.

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