refactor(v2)!: expand parameters and derive routes from protocols

[frostming]

Summary

V2 connection methods and agent/client handlers now accept expanded snake-case parameters, matching the v1 calling style. For example, connection.new_session(NewSessionRequest(cwd="/workspace")) becomes connection.new_session(cwd="/workspace"). Nested values and responses remain v2 schema models; extra keyword arguments carry _meta.

Add public v2.Agent and v2.Client protocols and derive routing specifications from their param_model metadata and return annotations. V2 retains separate contracts because initialization, prompt responses, permissions, and session updates differ from v1.

Preserve initialization gating, strict request/response validation, prompt acceptance acknowledgements, independent session updates, omitted-versus-null patch fields, union branch fields, separate MCP request/notification dispatch, and explicit extension names. Unimplemented protocol stubs remain unhandled. Signature generation now selects the appropriate schema version.

Breaking experimental API change: callers and handlers must migrate from whole request models to expanded parameters. Stable v1 APIs are unchanged. No schema bindings were regenerated.

Related issues

Follow-up to #149.

Testing

• make check — passed (lint/format, type checking, dependency checks).
• Python 3.10: UV_PROJECT_ENVIRONMENT=/tmp/acp-python-ci310 NO_PROXY=localhost,127.0.0.1 no_proxy=localhost,127.0.0.1 uv run --python 3.10 --frozen --all-extras python -m pytest --doctest-modules -q — 356 passed, 2 skipped.
• Python 3.14: NO_PROXY=localhost,127.0.0.1 no_proxy=localhost,127.0.0.1 uv run --frozen --all-extras python -m pytest --doctest-modules -q — 357 passed, 1 skipped.
• Regression coverage includes protocol route coverage, connection/handler signature agreement, union request round trips and metadata, strict v2 responses, inherited protocol stubs, patch field preservation, and schema-specific signature generation.

Docs & screenshots

Updated docs/experimental-v2.md with expanded calls, agent/client implementations, union request examples, and migration guidance.

Checklist

• Conventional Commit title.
• Tests cover the change.
• Docs/examples updated for the user-facing API change.
• Schema regeneration status called out above (none).