feat: add packaged live integration and provider regression coverage

[seratch]

This pull request adds a comprehensive packaged integration suite that validates the SDK from clean wheel and source-distribution installations before releases.

• Cover Responses and Chat Completions, typed and dictionary model settings, structured outputs, streaming, sessions, retries, approvals, handoffs, guardrails, tracing, and public import compatibility.
• Exercise hosted MCP, hosted multi-agent workflows, programmatic tool calling, code interpreter, web search, tool search, custom tools, and shell tools.
• Validate AnyLLM and LiteLLM integrations across configured OpenRouter-backed providers, including function tools, token usage, and logprobs.
• Verify Realtime sessions, tools, usage, handoffs, agent updates, audio guardrails, and voice pipeline behavior.
• Install and validate each optional dependency group independently.
• Provide release, nightly, manual, and focused execution profiles through dedicated Makefile targets and a repository-local $integration-tests skill.
• Keep the standard release workflow around six minutes by reserving expensive streaming, audio, concurrency, tracing, and compaction matrices for nightly runs.
• Exclude integration assets, repository automation, and local caches from published distributions.

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