feat: Add SearXNG as a selectable web_search provider

Problem statement / motivation

Part of #291
Status: Ready for implementation (builds on typed provider contract established in #293 and #294)
Scope: SearXNG provider integration, configuration plumbing, and tests

With You.com (commercial cloud) and Brave Search (free-tier cloud) in place, the gateway still lacks a fully self-hosted, on-premise, privacy-respecting search backend.

SearXNG is the open-source industry standard for self-hosted metasearch:

• Zero data leakage / On-Premise compliance: Ideal for enterprise and air-gapped deployments where user prompts and search queries must never leave the local network.
• No API key required: Connects directly to an internal or self-hosted SearXNG container instance.
• Metasearch aggregation: Capable of aggregating results across dozens of engines (Google, Bing, DuckDuckGo, Wikipedia) in a single normalized query.
• Differences from You.com / Brave that must be adapted: endpoint requires mandatory base URL (no single public endpoint), queries use format=json, category selection maps to news/web, and domain filtering is post-filtered client-side.

Proposed solution

A. Provider Implementation (tool/web_search/searxng.rs)

• Endpoint: GET {base_url}/search?q={query}&format=json&categories={categories}.
• Authentication: Optional. By default SearXNG requires no auth; optionally supports Bearer token or basic auth if behind a proxy via SEARXNG_API_KEY.
• Deserialization: Minimal forward-compatible structs with #[serde(default)] matching SearXNG's { "query": "...", "results": [ ... ] } JSON envelope.
• Transport Rule: Ensure Accept: application/json is sent; preserve gzip exclusion rule (core reqwest lacks gzip).

B. Input Adaptation Policy Matrix

C. Concurrency & Rate Limiting

• Concurrency Ceiling: Operator-governed. Inherits max_concurrent_gateway_calls by default, or configurable via AGENTIC_WEB_SEARCH_MAX_CONCURRENT_QUERIES.
• Error Handling: Non-2xx and non-JSON responses (e.g., if JSON format is disabled on the SearXNG instance) report actionable configuration errors guiding the operator to enable format=json in SearXNG's settings.yml.

D. Configuration & Precedence Matrix

Scope Boundaries

Verification Checklist

• 200 OK (Web + News): Verify SearXNG JSON envelope maps cleanly to WebSearchResult.
• Missing Base URL: Startup / call fails with actionable message explaining SearXNG requires a configured endpoint.
• Malformed / Disabled JSON: If SearXNG returns HTML instead of JSON, fail with diagnostic recommending search.formats: [html, json] in settings.yml.
• Domain Filtering: Verified client-side via DomainFilter.
• Freshness: Verified time_range translation.
• Metadata: Echoes "provider": "searxng" in metadata[].
• Zero Regressions: You.com and Brave Search remain byte-identical and unaffected.
• Hygiene: cargo fmt, cargo clippy --all-targets -- -D warnings, and file size limits pass.

Start with the typed provider contract established in #293 and #294, then read tool/web_search/searxng.rs and the surrounding provider configuration. Run or add Axum mock coverage in tests/web_search_searxng_test.rs for the JSON envelope, base URL and filtering mappings. Done means the verification checklist passes without regressions to You.com or Brave Search.