feat(server): introduce ServeConfig dataclass to consolidate serve() kwargs

[bokelley]

Closes #414

Summary

• Adds @dataclass(frozen=True) ServeConfig to src/adcp/server/serve.py, grouping all 22 serve() keyword arguments into a single named, IDE-friendly object with the same defaults
• serve() gains config: ServeConfig | None = None; when supplied, config fields win and individual kwargs are ignored (backwards-compatible — existing call sites work unchanged)
• __post_init__ emits UserWarning on cross-transport field combinations (A2A-only fields set alongside MCP transport and vice versa); stacklevel=3 empirically verified to point at the ServeConfig(...) call site
• Sequence moved from TYPE_CHECKING to runtime imports (fixes get_type_hints compatibility)
• Exported from adcp.server and added to __all__
• examples/typed_handler_demo.py updated to demonstrate the new pattern
• src/adcp/decisioning/serve.py docstring updated to surface config= in **serve_kwargs for agent discoverability
• 12 new tests in tests/test_serve_config.py; all 3200 existing tests pass

What tested

• ruff check — clean
• mypy src/adcp/server/ — no new errors (all pre-existing from optional deps not installed locally)
• pytest tests/test_serve_config.py — 12/12 passed
• pytest tests/ (excluding pre-existing network flake test_real_tls_handshake_still_validates_hostname) — 3200 passed, 0 new failures

Pre-PR review

• code-reviewer: approved — blocker addressed: unconditional test assertions replacing vacuous if call_count: guards; dead helper removed; stacklevel=3 confirmed correct via empirical test (stacklevel=4 overshoots to sys)
• dx-expert: approved — config= added to decisioning.serve() **serve_kwargs docstring for agent discoverability; note: DX expert's stacklevel=3→4 recommendation was empirically incorrect and reverted

Known nit (not fixed): SkillMiddleware and ContextFactory are defined after ServeConfig in the file. Under from __future__ import annotations (PEP 563) this is safe at runtime since the full module loads before any caller runs. Reorganising the ~400-line SkillMiddleware docstring block is out of scope for this PR.

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See adcp#3121
for context.

Session: https://claude.ai/code/session_016DD1aTKm7E12tgrceKdz1M