fix(search): update stale chain-tail copy and backend list

[asto18089]

Context

No-Issue: the CodeWhale repo tracks no issues; this batch is tracked as Pinvou/pinvou-agent#377 (r12 review leftovers).

Leftovers from the r12 release review (Pinvou/pinvou-agent#375, tracked as Pinvou/pinvou-agent#377): after #35 moved the keyless fallback tail for API-backed [search] providers from DuckDuckGo straight to Bing, three foundation-side spots still describe the old DuckDuckGo hop or an incomplete backend list. Docs/diagnostics only — no behavior change.

Changes

1. Model-visible web_search description (crates/tui/src/tools/web_search.rs:106): no longer claims "Configured API backends visibly degrade through DuckDuckGo then Bing"; now states the actual behavior — a visible degrade through the keyless Bing tail — matching the wording already used in crates/tui/src/config/search.rs.
2. Module doc (web_search.rs:1-7): the chain is now described as provider-native search, configured API search, then a keyless Bing tail chosen by reachability; API outages never route through DuckDuckGo, while the configured DuckDuckGo route keeps its own one-way Bing scrape fallback.
3. All-backends-down suggestion (crates/tui/src/tools/web/backend.rs:264): now lists all seven selectable API-backed providers (tavily, bocha, metaso, searxng, baidu, volcengine, sofya) — searxng and sofya were missing, although tools/spec.rs and docs/CONFIGURATION.md both count them as API-backed.
4. CHANGELOG: adds the missing \[Unreleased\] / Fixed entry for the fix(search): API 后端失败兜底从 DuckDuckGo 换成 Bing #35 chain-tail switch.

Not included

The optional DuckDuckGo image-search Bing fallback in web_run.rs (item 4 of the tracking issue) is a new scraping adapter with its own parsing surface and tests, not a copy/diagnostics cleanup; left out of this PR to keep the batch reviewable.

Verification

• cargo test -p codewhale-tui --lib tools::web::backend — 10 passed, including forkguard_api_provider_chain_tail_is_bing and the all-backends-down error-message test.
• cargo test -p codewhale-tui --lib tools::web_search — 66 passed.
• cargo fmt -p codewhale-tui -- --check — clean.
• Parent-repo ./scripts/fork-guard.sh --fast — all fingerprint layers pass; the only two failures are the layer-0 gitlink topology check, which resolves when the next baseline batch registers this commit (the tracking issue defers the parent gitlink to the next baseline batch).

Signed-off-by: asto18089 asto18089@126.com

[github-actions]

Thanks @asto18089 for taking the time to contribute.

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