docs: use the current You.com Search API host in config examples

[tyler5673]

Summary

The README's config.toml example sets the You.com web search base_url to https://api.ydc-index.io, the You.com API's legacy host. That host returns 403 {"message":"Forbidden"} for API keys created at you.com/platform since November 2025, so a deployment that copies the README example can't search. This switches the example to https://ydc-index.io, the host in You.com's API docs. docs/deploying/kubernetes.md already uses it.

It also updates the same value in two config tests (creates_secret_free_default_config_from_runtime_settings and the typed-config.toml fixture). The default_base_url behavior is unchanged: You.com still has no built-in default, per #291.

This partly answers #291 Q2 ("which host is canonical"): it's https://ydc-index.io. I work at You.com.

Test Plan

• Live check of the unchanged YouSearchProvider with a current You.com key, through a temporary ignored test (not included in this PR):
  • base_url = https://api.ydc-index.io: You.com search returned 403 Forbidden: {"message":"Forbidden"}
  • base_url = https://ydc-index.io: 3 web results (github.com/vllm-project/vllm, vllm.ai, en.wikipedia.org/wiki/VLLM)
• cargo test: all pass, including the 27 config_file::tests
• cargo clippy --all-targets -- -D warnings: clean
• cargo fmt -- --check and pre-commit run on the changed files: pass

AI assistance (Claude) was used to prepare this change. I reviewed and tested it.