feat(providers): opt in to provider-hosted web search per model

[zeroy1024]

Summary

Adds an opt-in, per-model native web search capability: when enabled in the model's settings (attachment section), the provider's server-side search tool is attached on the wires that define one — web_search_20250305 for anthropic-messages, {type:"web_search"} for openai-responses / azure-openai-responses — and each search round renders as its own transcript activity row, live while streaming.

Unlike an MCP search server, a hosted tool executes inside the provider request and returns structured blocks that must round-trip for multi-turn grounding, so extraction and replay live in the adapters rather than in a plugin (rationale in ADR 0294).

How it works

• Data: ModelBinding.nativeWebSearch (two-state opt-in; no catalog baseline since models.dev publishes no hosted-tool capability) flows through the existing model-config channel into pi-ai's Model.webSearch. One evaluation point, resolveNativeWebSearch in @pi-desktop/shared, keyed on the resolved wire API only — never vendor names or URL hostnames.
• Wire (patches/@earendil-works__pi-ai@0.85.1.patch): attach the tool; ask Responses for web_search_call.action.sources via include; extract server_tool_use / web_search_tool_result / web_search_call / citation annotations into hostedSearch content blocks (raw wire payloads kept verbatim); accumulate streamed input_json_delta so the query is visible live; replay both block halves on later turns. The Azure Responses variant gets the same attachment.
• Streaming (patches/@earendil-works__pi-agent-core@0.85.1.patch): the agent loop forwards the adapters' hosted_search_update as message_update; without it the events died in the loop's switch and search activity only appeared when the turn finished.
• Runtime/UI: hostedSearchFromBlocks normalizes to UiMessage.hostedSearch.rounds[] (one round per provider call, paired by block id, per-round status/query/sources). Each round renders as a HostedSearchRow on the standard tool-row idiom, distinguishing search / open-page / find-in-page. Sources are plain text links — no favicon fetches, so transcripts never leak source hostnames to third parties (fix(desktop): keep hosted-search citations on the source origin #579 rule).
• Persistence: additive hostedSearch transcript block via ui_to_record/record_to_ui; no SQL migration.

Affected docs

• ADR 0294 (new): provider-hosted web search as an adapter capability
• docs/spec/03-runtime/11-provider-model-system.md (rule 10a), 04-data-storage.md (block vocabulary) — both languages

Validation

• pnpm build:js, workspace typecheck (0 errors), pnpm lint (biome + style tokens), architecture check
• JS suites: shared 818 · agent-runtime 620 (incl. new stream/params contract tests and an agent-loop forwarding regression test) · desktop 2320 · i18n 25 · all other packages green
• cargo test -p host-core --locked: 525 passed; cargo fmt --check; clippy clean apart from one pre-existing unrelated warning
• Live wire-compatibility matrix (21 requests, all HTTP 200) across a third-party gateway and DeepSeek official endpoints, both wires: bare tool accepted everywhere; include safe everywhere; Kimi channels only return sources via include; DeepSeek's Anthropic wire executes real server-side search; DeepSeek's Responses wire accepts but never executes (no search rows, pseudo-call text logged by the existing pseudo-tool-call detector)
• E2E harness: NOT RUN (no cross-process risk beyond the covered contracts; streaming path covered by the new agent-loop forwarding test)

Compatibility & risks

• Default off; zero behavior change for existing providers/models.
• Endpoints that accept but never execute the tool degrade to no search rows (no silent fake success); gateways that mangle replay blocks surface as provider errors on the next turn, remedied by unchecking the model's opt-in.
• The two dependency patches are stopgaps meant for upstream submission to @earendil-works/pi-ai / pi-agent-core and removal once released.
• pause_turn keeps pi-ai's existing mapping to stop; a long searching turn on the official Anthropic wire may end early. Known limitation, noted in the ADR.

[vastsa]

Merged onto current main with a restart-replay fix in #635 (ADR renumbered to 0296 because 0294 was already taken). This PR can close once #635 lands.