fix(server): strip None-valued asset fields from dict-based response builders

[bokelley]

Closes #622

Summary

ImageAsset, VideoAsset, and UrlAsset have optional Python fields (format: str | None = None, alt_text, provenance, container_format, etc.) that default to None. The bundled JSON schemas declare these fields as non-nullable ("type": "string", not ["string", "null"]). When adopters build dict-based creative responses with those fields set to None, the null values reach the wire and cause the storyboard schema validator to reject the asset — the oneOf discriminated union branch fails because null is not a string per JSON Schema Draft 7.

The fix adds _strip_none_values() — a recursive helper that removes None-valued keys from dicts before wire serialisation — and applies it in:

• _serialize() — for dict items, after the existing _strip_write_only_fields() pass
• sync_creatives_response() — now routes through _serialize() (was returning raw list)
• preview_creative_response() — now routes through _serialize() (was unguarded)
• build_creative_response() — for both single and multi-manifest variants

The Pydantic model path was already safe: _serialize() calls model_dump(mode="json", exclude_none=True) for model items, and all SDK dispatch sites (serve.py, a2a_server.py, mcp_tools.py) also pass exclude_none=True. This PR closes the gap in the dict (loose-dict / hand-built payload) path, which is the most common pattern in adopter code.

What tested

• ruff check src/adcp/server/responses.py tests/test_server_dx.py — passes
• uv run mypy src/adcp/server/responses.py — passes (no issues in 1 source file)
• uv run --all-extras pytest tests/test_server_dx.py -v — 56/56 passed
• Full suite pytest tests/ — 128 passed, 1 pre-existing failure (test_real_tls_handshake_still_validates_hostname — network env issue, unrelated to this change)
• 12 new tests added covering _strip_none_values unit behaviour and list_creatives_response, preview_creative_response, build_creative_response, sync_creatives_response with explicit None-valued ImageAsset and VideoAsset fields

Nit noted (not fixed): sync_creatives_response() wiring through _serialize() also adds the write-only-field strip to sync records — this is safe but slightly broader than needed; flagging for reviewer awareness.

Upstream note: Widening the JSON schema to ["string", "null"] for these asset fields (so that nullable-in-Python == nullable-on-wire) is the cleaner long-term fix and would require a spec-level change in adcontextprotocol/adcp. That is tracked separately.

Pre-PR review

• code-reviewer: approved — blockers (sync_creatives, preview Pydantic path) addressed in final diff
• ad-tech-protocol-expert: approved — strip-rather-than-null matches AdCP/OpenRTB "omit rather than null" convention; no spec-defined sentinel use case for null on these fields

Triage-managed PR. This bot does not currently iterate on
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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_01AkZRebtF7xsZJZ2FPnQmNL

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