refactor: promote history compaction to a first-class pydantic-ai capability

[mpfaffenberger]

What

The in-run compaction pipeline was an async closure (make_history_processor) wrapped in the generic ProcessHistory adapter at two call sites (main builder + sub-agent invocation). This promotes it to HistoryCompaction, a proper AbstractCapability subclass living in _compaction.py — the same house-style upgrade #828 gave the steering injector.

Why

• The builder already composes capabilities=[...]; compaction was the biggest feature still hiding behind a closure-in-an-adapter.
• DRY: HistoryCompaction(agent) replaces closure construction + ProcessHistory wrapping at both call sites.
• The capability owns its docs: the 7-step pipeline contract and the agent attribute contract now live on the class that implements them.

Design notes

• Byte-identical seam semantics. before_model_request REPLACES request_context.messages with the processed durable history — exactly what ProcessHistory does internally — so registration order against steer injection and the response clamp is preserved (compaction still fires first; a fresh steer can't be compacted away).
• get_serialization_name() -> None — not spec-serializable, it holds a live agent reference (same rationale as ProcessHistory itself).
• Feature parity checklist: start/end history-processor hooks hash-dedup merge with newest-message collision protection forced mid-run /compact via pause controller dropped-hash bookkeeping empty-ThinkingPart stripping trailing-ModelResponse trim (Anthropic prefill) tool_call_id sanitization spinner context badge updates

Tests

• Ported the closure suite to drive the real before_model_request seam.
• New: seam-parity test (messages replaced, not appended) + a TestModel-backed Agent.run() proving dispatch through the actual capability chain lands the user prompt in the durable history.
• Removed two stale make_history_processor mock patches — constructing the dataclass is side-effect-free.
• Full suite: 7580 passed; the one failure (test_render_version_check_current) is pre-existing on main.

Heads-up

Touches the same capabilities=[...] block as #828 (steering capability). Whichever lands second gets a trivial rebase.

Do not merge — awaiting review.