docs(changelog): record keyless Bing search tail

[asto18089]

Context

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

Rebased onto pinvou3-clean @ 1fafee7e2 (post-v0.9.12). During the rebase, the three copy/diagnostics fixes this PR originally carried turned out to be already absorbed upstream in evolved, tested form, so the branch now carries only the piece still missing there.

Changes

1. CHANGELOG: adds the missing [Unreleased] / Fixed entry for the keyless chain-tail switch, adapted to the current chain: API-backed [search] providers degrade directly to the keyless Bing tail without a DuckDuckGo hop, the all-backends-down guidance names every keyed provider (tavily, bocha, metaso, baidu, volcengine, sofya) plus the keyless routes (firecrawl, bing), and the web_search tool description no longer claims a DuckDuckGo hop.

Where the original items went

• Model-visible description + module doc (crates/tui/src/tools/web_search.rs): upstream rewrote both in ff299f94b ("fix(search): preserve reachable API fallback") with the same meaning against the newer chain — "visibly degrade directly to keyless Bing", keyless Firecrawl default — pinned by forkguard_api_provider_chain_tail_is_bing.
• All-backends-down suggestion list (crates/tui/src/tools/web/backend.rs): superseded by 1fafee7e2 ("fix(search): restore actionable backend guidance"), whose SEARCH_BACKEND_CONFIGURATION_HINT adds sofya and splits keyed from keyless guidance. SearXNG intentionally sits on the keyless/base_url side there; if reviewers want it named explicitly, that is a one-line follow-up on top of 1fafee7e2.

Not included

The branch no longer carries the fork-lineage commits that rode along before; they stay queued via their own PRs (#31, #34, #37, #38, #41, #43, #47).

Verification

• Diff vs pinvou3-clean is CHANGELOG.md only (+12 lines): no behavior or build-surface change; the upstream CI gates run on this PR.
• The behavior the entry describes is already pinned upstream by forkguard_api_provider_chain_tail_is_bing and the configuration-hint tests added in 1fafee7e2.

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

[github-actions]

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