feat(pydantic-acp): auth recovery, public session bootstrap, and clean agent-error propagation

[arthrod]

Description

Hardens the pydantic-acp ACP client/provider so an unusable ACP agent fails legibly instead of silently, and adds the session-bootstrap surface the bridge was missing. Single commit off main; ships as synchronized release 1.5.0 → 1.6.0.

Motivation: driving real stdio ACP agents (Claude Code, GLM, dirac, vibe, grok, …) surfaced three gaps where a broken/unauthenticated/throttled agent produced an empty turn or an opaque ExceptionGroup with no indication of the real cause, and no public way to bring a session up before prompting.

Added

• Auth recovery. AcpProvider captures initialize's auth_methods and, when session/new is rejected with auth_required (-32000), runs the ACP authenticate flow — an advertised method, or an explicit auth_method_id= — and retries session creation once. Agents that gate session/new on authentication were previously unrecoverable.
• Public session bootstrap. AcpProvider.ensure_session() and AcpProvider.set_session_mode() bootstrap a session and select a session mode without sending a prompt turn, so callers no longer reach into the private _ensure_session.
• Empty-turn diagnostic. Opt-in AcpProvider(raise_on_empty_turn=True) raises UnexpectedModelBehavior with an ACP-specific message when a text-output turn yields no visible text. Default False preserves the existing empty-response contract (and its test).

Fixed

• Error propagation. request_prompt propagates the agent's real error (rate limit, auth rejection, upstream API failure) by unwrapping single-child anyio TaskGroup BaseExceptionGroups and dropping TaskGroup __context__ noise, instead of surfacing an opaque ExceptionGroup: unhandled errors in a TaskGroup. asyncio.CancelledError is re-raised untouched.

Fixes # (n/a)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project (ruff format & check)
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (CHANGELOG.md [1.6.0])
• My changes generate no new warnings (basedpyright & ty)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes (pytest)

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Verification

• make check — ruff, ty, basedpyright all clean
• make tests — 694 passed
• make check-coverage — thresholds satisfied; pydantic_acp/client.py at 100% line + branch (added an authenticate/ensure_session/set_session_mode/raise_on_empty_turn/unwrap/CancelledError test set)
• uv run pre-commit run --all-files — all hooks pass, including version-ahead-of-PyPI
• Synchronized 1.6.0 bump via bump.sh (version files + root extras) + CHANGELOG entry

🤖 Generated with Claude Code

[fswair]

I'll check today, thanks for contribution.