Studio: standalone Fetch pill for Anthropic web_fetch (#5742)

* Studio: surface Anthropic web_fetch as a standalone Fetch pill

web_fetch used to be silently bundled with the Search pill on the
assumption that "search returns URLs, fetch reads them" is the
typical workflow. Two problems with that:

- Anthropic bills each web_fetch invocation separately from
  web_search hits, so combining them made the per-message cost
  surface ambiguous.
- It blocked "just fetch this one URL" workflows where the user
  already knows the page they want read and does not want a search
  round-trip.

Adds:

- `webFetchToolsEnabled` to the chat-runtime-store, persisted to
  localStorage under `unsloth_chat_web_fetch_tools_enabled`, with a
  matching `supportsBuiltinWebFetch` capability flag and a
  `setWebFetchToolsEnabled` setter.
- A new Fetch pill in the chat composer, rendered next to Images and
  only when the active provider returns true from
  `providerSupportsBuiltinWebFetch` (Anthropic today). The pill
  defaults off so per-fetch billing is always a deliberate opt-in.
- chat-page bootstraps `webFetchToolsEnabled` from the same stored-
  preference fallback the other pills use.
- chat-adapter reads `webFetchToolsEnabled` directly when deciding
  whether to append "web_fetch" to `enabled_tools`, decoupling it
  from `toolsEnabled` (Search).

Backend translation is unchanged: when `enabled_tools` already
contains "web_fetch", `_stream_anthropic` appends the
`web_fetch_20250910` / `web_fetch_20260209` tool exactly as before
(test_anthropic_web_fetch.py pins the standalone-only path at
`test_web_fetch_tool_appended_to_request_body` and the combined
path at `test_web_fetch_combined_with_web_search_and_code_execution`).
Frontend tsc passes.

* ci: re-trigger after transient GitHub API HTTP flake (checkout + ggml-org release fetch)

* Studio: include web_fetch in the disabled-tool guard axis

Reviewer P1 / High on PR #5742 (codex + gemini): after introducing
the standalone Fetch pill, `disabledToolGuard` still only branched on
`webSearchEnabledForThisTurn`. With Fetch ON and Search OFF the
system prompt would tell Claude "you do not have web search or web
fetch tools in this conversation", which contradicts the actual tool
schema being sent and suppresses `web_fetch` tool calls, defeating
the standalone-fetch workflow this PR adds.

Treat search and fetch as a single "any web tool enabled" axis. The
guard only needs to warn the model when no web tool is wired in for
this turn; once either pill is on the model can pick the right one
from the tool schema. The existing `webLabel` already covers both
names, so the user-visible guard text stays accurate in every
combination.

tsc clean.

* ci: re-trigger after transient infra flake on Windows prebuilt / actions/checkout

* Studio: route web_fetch through per-model version dispatch

The web_fetch tool body in `_stream_anthropic` hardcoded
`web_fetch_20250910` instead of calling `_anthropic_web_fetch_version`,
so Opus 4.6 / 4.7 and Sonnet 4.6 missed the `web_fetch_20260209`
dynamic-filtering variant. The picker, the unit tests for it, and a
deliberate "follow-up" note in `test_anthropic_web_fetch.py` already
existed; this just threads it through the emission site.

Mirrors how web_search and code_execution are dispatched per model.
Old models still resolve to `web_fetch_20250910` and continue to work.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Shorten web_fetch comments for PR #5742

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