feat(advisor) 2/4: preserve advisor blocks in ChatResponse round-trip

[ericleepi314]

2/4 in the /advisor stack. Stacked on #176.

Summary

The Anthropic provider's _build_chat_response projects the SDK stream into (content, tool_uses, ...) for the agent loop, but drops everything else — including server-side advisor blocks. Once PR 3 wires activation, those blocks need to round-trip through history so the next turn's API call sees a matched server_tool_use(name=advisor) + advisor_tool_result pair.

• src/providers/base.py — adds ChatResponse.raw_content_blocks: Optional[list[dict]] = None, the channel for passthrough blocks the projector chose not to flatten.
• src/providers/anthropic_provider.py — extends _build_chat_response to collect blocks where type == 'advisor_tool_result' or (type == 'server_tool_use' and name == 'advisor'), serialize via block.model_dump(exclude_none=True), and attach to raw_content_blocks. The SDK's lenient construct_type preserves the original fields on unknown discriminators (verified empirically against anthropic==0.88.0).

Other server tools (web_search, code_execution, etc.) are deliberately NOT scooped up — they have their own SDK projection.

Stack

• PR 1 (feat(advisor) 1/4: helpers + settings field + orphan tool_use guard #176) — helpers + settings + orphan guard
• PR 2 (this PR) → feat(advisor) 1/4: helpers + settings field + orphan tool_use guard #176
• PR 3 (query wiring) → this branch
• PR 4 (slash command + TUI) → PR 3

Test plan

• tests/test_advisor_chat_response_roundtrip.py — 6 tests: builds a real SDK ParsedMessage via accumulate_event for each discriminated advisor_tool_result content shape (advisor_result, advisor_redacted_result, advisor_tool_result_error), the orphan case, the non-advisor server-tool exclusion, and a mixed-block case.
• Pins the SDK round-trip contract — a future SDK upgrade (Pydantic v3, stricter discriminator handler) that silently drops extra fields on unknown discriminators would fail the encrypted_content and error_code assertions locally.

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