fix(compat): warn on non-standard brand_manifest path in inline-object branch

[bokelley]

Summary

Fixes #687. After #685 (inline-object branch) and #686 (non-standard-path warning) landed, the warning was applied only to the URL-string branch. The v2.5 brand-manifest-ref schema allows brand_manifest to be either a URL string or an inline BrandManifest object — the inline object's url field carries the same path semantics, and the same information loss applies (v3 sellers fetch the canonical {domain}/.well-known/brand.json and will likely 404).

Changes

• src/adcp/compat/legacy/v2_5/_url.py: new warn_brand_manifest_path_lossy(manifest_url, domain) function plus the _STANDARD_BRAND_MANIFEST_PATHS constant and _brand_manifest_path_warned dedup set (moved from per-adapter modules). Logger is adcp.compat.legacy.v2_5._url.
• src/adcp/compat/legacy/v2_5/get_products.py and _media_buy_helpers.py: remove the per-module helpers; call warn_brand_manifest_path_lossy from both the string and inline-object branches.
• Tests updated for the new monkeypatch target and logger name. Four new regression tests cover the inline-object warning behaviour (CDN url warns, canonical url silent) on both adapters.

Test plan

• pytest tests/test_legacy_adapter_v2_5_*.py — 64 passed (was 60; +4 new dict-branch warning tests)
• Verified the two new tests fail against pre-fix source with assert 0 == 1 (no warning emitted)
• ruff check and mypy src/adcp/compat/legacy/v2_5/ clean
• Black formatted (pre-commit re-ran)

Logger-name change

Operators previously alerting on adcp.compat.legacy.v2_5.get_products / adcp.compat.legacy.v2_5._media_buy_helpers warnings will need to update the logger name to adcp.compat.legacy.v2_5._url. The warning message text is unchanged, so grep-based alerting on "non-standard path" is unaffected. Acceptable shift since #686 landed only an hour ago and adoption of log-based alerting on this specific warning is unlikely.

[bokelley]

Expert review

code-reviewer: approved — no blockers. Confirmed:

• Refactor preserves behaviour exactly (same three-clause guard, same message format, same dedup semantics, same WARNING level)
• Both call sites symmetric after refactor
• 4 new regression tests well-scoped
• Logger-name change is a real (small) breaking shift but blast radius is near-zero since fix(compat): warn when brand_manifest non-standard path is silently flattened to domain #686 landed ~1h ago — fine as-is

Non-blocking nits filed for future work:

• Pytest autouse fixture for _brand_manifest_path_warned reset would prevent future test pollution (currently every test monkeypatches; one missed reset would flake)
• Optional cross-adapter dedup test would document the now-shared-set semantic across get_products and create_media_buy

CI: 15/15 green. Merging.