docs(examples): expand hello_seller.py to full 9-method sales-non-guaranteed surface

[bokelley]

Closes #515

Summary

hello_seller.py implemented only the five hard-required sales-non-guaranteed methods, causing validate_platform to soft-warn about four additional SalesPlatform Protocol methods staged for hard-required status at v6.0 rc.1 GA (get_media_buys, list_creative_formats, list_creatives, provide_performance_feedback). A canonical example that emits warnings at boot teaches adopters to ignore validator signal — the opposite of the fail-fast contract.

Decision: Expand the example (option 1 from the issue). The four methods are in RECOMMENDED_METHODS_PER_SPECIALISM on an explicit promotion path to REQUIRED_METHODS_PER_SPECIALISM; the example should reflect the full required shape that adopters will need.

Changes:

• Add stub implementations for all four soft-required methods, each returning the minimal valid wire shape with clear docstrings directing adopters to wire production systems
• Update module and class docstrings: correct "five required" → "nine required", add hard/soft distinction ("soft-required", pointer to RECOMMENDED_METHODS_PER_SPECIALISM), fix stale "3 implemented" → "9 seller methods" in list-tools walkthrough
• Add dispatch-path smoke tests for all four new stubs + a validate_platform assertion confirming HelloSeller passes with zero soft-warns

What tested

• pytest tests/test_hello_seller_integration.py — 12 passed (7 pre-existing + 5 new)
• Full suite: pytest tests/ -m "not integration" — 3759 passed, 0 regressions (1 pre-existing TLS environment failure unrelated to this change: test_real_tls_handshake_still_validates_hostname)
• validate_platform(HelloSeller()) — zero soft-warns for the four recommended methods
• python -c "import ast; ast.parse(open('examples/hello_seller.py').read())" — syntax OK

Pre-PR review

• code-reviewer: approved — no blockers after second pass. Found and fixed: stale "3 implemented" count, D1 projection signatures verified correct (all four use standard (req, ctx)), test coverage gap → addressed with 5 new tests.
• dx-expert: approved direction — stubs clearly annotated as wiring targets, section comment uses "recommended (rc.1+ staging)" language consistent with RECOMMENDED_METHODS_PER_SPECIALISM naming.
• docs-expert: approved — module docstring correctly distinguishes hard/soft with "hard-fail for the five, soft-warn for the four"; section comment updated to "recommended" to match map name; ADCP_DECISIONING_STRICT_VALIDATE_PLATFORM=1 CI guidance added.

Nits surfaced (not fixed — PR scope):

• sales.py per-method docstrings for the four methods still say "Required … validate_platform fails server boot" (hard-required language). These live in the Protocol file, not the example. Separate cleanup issue warranted.

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_01SQEG3PCARK4eKHBbqu4fTS

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