feat(migrate): flag MediaBuyStatus.pending_activation split in v3-to-v4 codemod

[bokelley]

Refs #513

Adds REMOVED_ENUM_VALUES dict and a flag_enum_value finding kind to the v3→v4 codemod so adopters migrating from v3.12 see an actionable codemod report entry for MediaBuyStatus.pending_activation rather than a runtime AttributeError. The hint names both replacement values (pending_start / pending_creatives) and points to valid_actions on the MediaBuy response as the runtime discriminator for which applies at a given call site.

Item 2 from #513 (required-field detection for idempotency_key) is deferred — text-match is not reliable for multi-line constructor calls and **kwargs unpacking; an AST-based pass is the correct architecture for that.

What was tested

• pytest tests/test_migrate_v3_to_v4.py -v — 35 passed (2 new: test_flags_removed_enum_values, test_enum_value_word_boundary_no_false_positive)
• ruff check src/adcp/migrate/v3_to_v4.py — clean
• mypy src/adcp/migrate/v3_to_v4.py — no errors in changed file

Pre-PR review

• code-reviewer: approved — no blockers; non-breaking (additive new dict + scan loop, existing findings unchanged); nit: kind field could use Literal type (out of scope)
• dx-expert: approved — flag_enum_value as separate kind is correct for JSON consumers; hint is actionable; nit: hint opens redundantly with enum value name already shown in header; no migration_anchor slot (noted, acceptable until migration doc adds a section for it)

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> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_01294Agd3ZTguC1drs5Beous

---

Generated by Claude Code