feat(model): add scripted chat model for tests

[March-77]

What type of PR is this?

feat

Check the PR title.

• This PR title matches the format: <type>(optional scope): <description>.
• The description is user-oriented and clear enough for others to understand.
• The new testing API is documented inline and includes a runnable example.

(Optional) Translate the PR title into Chinese.

feat(model): 添加面向单元测试的可脚本化 ChatModel

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:

Eino users currently need to implement BaseChatModel manually or depend on gomock to test multi-turn agents and graphs. The generated mock under internal/ is intentionally unavailable to downstream applications.

This PR adds a dependency-free components/model/modeltest package with:

• fluent turn-based scripts through OnTurn;
• content-aware scripts through OnPrompt matchers;
• complete messages, tool calls, errors, and scripted stream chunks;
• matching Generate and Stream behavior, including concatenating stream chunks for Generate;
• concurrency-safe call history with defensive message, tool, and common-option snapshots;
• helper assertions such as LastUserMessage and LastToolMessageName;
• immutable WithTools derivation implementing model.ToolCallingChatModel;
• a runnable package example.

User impact: applications can unit-test ChatModelAgent, ReAct, and graph behavior without provider credentials, network calls, gomock setup, or handwritten model stubs.

Compatibility/risk: this is an additive package with no changes to existing interfaces or implementations. Each scripted expectation is consumed once, making missing or unexpected calls fail deterministically.

Validation:

• go test ./components/model/modeltest -count=1 (pass)
• go test -race ./components/model/modeltest -count=1 (pass)
• go test ./... -count=1 (pass)
• go vet ./... (pass)
• gofmt -s and git diff --check (pass)
• golangci-lint could not start because its first-time v2.12.2 download repeatedly stalled on github.com/MirrexOne/unqueryvet; both attempts were stopped after no progress.

zh(optional):

新增无外部依赖的可脚本化 ChatModel，支持按轮次或 Prompt 匹配响应、流式分片、工具调用、错误响应、调用历史及不可变工具绑定，供下游 Agent 和 Graph 单元测试直接使用。

(Optional) Which issue(s) this PR fixes:

Fixes #1029

(optional) The PR that updates user documentation:

N/A