feat(decisioning): time_budget deadline wrapper + incomplete projection on get_products

[bokelley]

Closes #495

Summary

Honors GetProductsRequest.time_budget in MediaBuyHandler.get_products. Previously the SDK ignored the field entirely; the platform ran unbounded. Now:

• resolve_time_budget({interval, unit}) converts the wire Duration to a float seconds deadline; unit='campaign' returns None (no SDK-managed deadline — seller has the full campaign flight; value still reaches the adopter via params).
• The get_products shim wraps _invoke_platform_method in asyncio.wait_for(coro, timeout=deadline) (only when deadline is not None).
• On asyncio.TimeoutError: logs a WARNING with budget info + returns {"products": [], "incomplete": [{"scope": "products", "description": "…", "estimated_wait": null}]}.
• IncrementalGetProducts Protocol + ProductsCheckpoint declared and exported from adcp.decisioning; dispatch routing ships in a follow-up (documented in AGENTS.md and the Protocol docstring).

Behavioral change for existing adopters: Any adopter whose get_products latency exceeds a buyer's time_budget will now see their coroutine cancelled and buyers will receive incomplete[]. Previously the field was silently ignored. A WARNING log fires on each timeout — monitor for regressions after upgrade. The thread-pool slot leak for sync adopters (thread continues post-cancel) is documented in the time_budget.py module header.

Nits not fixed (noted for follow-up):

• Protocol expert: enumerating only scope='products' on full timeout — pricing/forecast scope descriptions in the spec presuppose products existing, so products: [] makes them inapplicable; proposals scope enumeration deferred to the incremental dispatch PR.
• Protocol expert: strip_credentials_from_wire_result not called on the SDK-generated timeout dict — the dict is known-safe (no adopter data), but worth adding defensively in the follow-up.

What was tested

• pytest tests/test_time_budget.py — 20 new tests, all pass: unit conversion for all 4 time units + campaign + None; project_incomplete_response shape; 1s budget vs 10s adapter → incomplete[]; within-budget passthrough; absent budget → no deadline; campaign unit → no deadline; timeout → WARNING log; Protocol importability.
• pytest tests/test_decisioning_handler.py tests/test_decisioning_handler_shims.py tests/test_decisioning_dispatch.py — 109 existing tests, no regressions.
• Full unit suite (pytest tests/ -x -q --ignore=tests/integration --ignore=tests/conformance) — 3210 passed, 0 failures.
• ruff check src/adcp/decisioning/time_budget.py src/adcp/decisioning/handler.py — clean.

Pre-PR review

• code-reviewer: approved — blockers resolved (IncrementalGetProducts dispatch status documented in AGENTS.md + Protocol docstring; description field present in incomplete dict; type annotations retained on shim)
• ad-tech-protocol-expert: approved with nits — scope='products' is spec-correct for products: [] case (other scopes presuppose products); campaign→no-deadline documented in module header + Protocol docstring; estimated_wait: None is schema-valid

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
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  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_01FMKCUEQWdb8my9rXtFVQ9x

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