Promote plugin message transform to a first-class capability

[mpfaffenberger]

What

Third entry in the capability-ification series (after #828 SteerInjection and #829 HistoryCompaction): the transform_model_messages plugin hook was a closure stuffed into the generic Hooks(model_request=...) adapter — at two call sites. It's now PluginMessageTransform, a @dataclass AbstractCapability subclass in code_puppy/agents/_model_message_transform.py.

Why

• Hooks(model_request=...) registers on the wrap_model_request seam (verified in pydantic_ai/capabilities/hooks.py — the model_request kwarg maps straight to 'wrap_model_request'). Overriding that method on a subclass is the same seam at the same list position, so ordering vs. history processors and the response clamp is byte-identical.
• Both call sites (agents/_builder.py and tools/subagent_invocation.py) now wire PluginMessageTransform(agent_name) directly — no factory closure, no adapter indirection. The build_model_message_transform factory is gone (YAGNI: it added nothing once the capability holds only a plain agent_name field).

Feature parity checklist

• Shallow-copy of ModelRequestContext + fresh list(...) of messages — plugin mutations are request-only and never leak into durable history (result.all_messages() stays clean)
• Async fan-out via on_transform_model_messages(agent_name, messages) — callback failure isolation unchanged (lives in the fan-out, not here)
• Runs after compaction/steering/clamp — same position in capabilities=[...] at both call sites
• Streaming path covered (existing TestModel streaming test still green)
• Serialization: unlike refactor: promote steer injection to a first-class pydantic-ai capability #828/refactor: promote history compaction to a first-class pydantic-ai capability #829, this capability holds no live agent reference — just a str | None — so it keeps the inherited get_serialization_name() default (spec-constructible)

Tests

• 2 new contract tests drive wrap_model_request directly: copy isolation (caller's context and list object untouched, handler receives the mutated fresh copy, sentinel response passes through) and bare passthrough with zero callbacks registered
• Existing suite updated to construct the capability directly; the FunctionModel-backed run test still proves post-history-processing ordering and request-only semantics on the real wire
• Full suite: 7490 passed; the one red (test_render_version_check_current) is pre-existing on main and unrelated

Notes

• Do not merge yet (per instructions).
• Independent of refactor: promote steer injection to a first-class pydantic-ai capability #828/refactor: promote history compaction to a first-class pydantic-ai capability #829 — this touches a different line of the shared capabilities=[...] block, so expect at most a trivial rebase whichever lands last.