Add explicitly-present web_fetch_enabled request setting

[warp-agent-staging]

Description

Adds Request.Settings.web_fetch_enabled so an Agent Profile can allow Exa web fetch independently of web search. Today web_search_enabled gates both built-in web tools.

The field uses explicit presence ([features.field_presence = EXPLICIT]), so the server can tell an old client (unset) from a profile that turns fetch off (explicit false). When the field is unset, the server falls back to web_search_enabled. That keeps existing profiles that disabled web search from gaining fetch.

Generated shapes:

• Rust (prost): web_fetch_enabled: Option<bool>
• Go (opaque API): GetWebFetchEnabled, HasWebFetchEnabled, SetWebFetchEnabled, ClearWebFetchEnabled; builder field *bool

Field number: this uses 37, not the next free number 36, because the open #393 already claims Settings field 36. Either PR can merge first without a renumber. This supersedes the stale #227, which uses field 26 (now supports_research_agent).

Related PRs: server profile split warpdotdev/warp-server#19137 and Warp client profile toggle warpdotdev/warp#16315. Merge order: this PR → warpdotdev/warp-server#19137 → warpdotdev/warp#16315.

Justification

Requested by the Agent Profile web-fetch toggle work: users want web search without page fetch (or the reverse), and team admins need to control each tool separately.

Sensitive Fields

No new string fields; the only new field is a bool.

Validation

• ./script/generate -a multi_agent -v v1 with protoc 36.0 and protoc-gen-go v1.36.6 (the CI versions): only request.proto and request.pb.go change.
• go build ./... in apis/multi_agent: passes.
• cargo build and cargo test: pass; the generated Rust has #[prost(bool, optional, tag = "37")] pub web_fetch_enabled: Option<bool>.
• npm ci && npm run generate:ts: passes; TS has webFetchEnabled: boolean.

There is no UI in this PR. The recordings and screenshots below come from the dependent Warp client PR warpdotdev/warp#16315.

Recordings

End-to-end evidence from the dependent Warp client PR warpdotdev/warp#16315 (real Linux app build). These show the Agent Profile UI and the persisted profile, not this PR's code rendering anything directly.

• Independent web search / web fetch toggles, closed and reopened
• Legacy profile without the fetch field falls back to web search

Screenshots

Before (baseline client build):

• Profile card
• Profile editor

After (changed client build):

• Profile card
• Profile editor
• Settings persist across restart
• Legacy profile fallback

[warp-agent-staging]

This PR was generated with Warp.

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