Add CLI validation tests for supported engines

[karust]

Context

Passing an unknown engine name currently fails with no "x" search engine found,
which doesn't tell the user what's valid. Browser mode and raw mode also support
different engine sets, and nothing pins that behavior in a test.

Scope

1. Make the unknown-engine error list the valid engine names for the mode.
2. Add a table test in cmd/ asserting:
   • browser mode accepts google, yandex, baidu, bing, duckduckgo, ecosia;
   • raw mode accepts google, yandex, baidu, ecosia and returns a clear
     error for bing and duckduckgo;
   • an unknown name returns the new listing error.

Done when

• The error message names the valid engines for the mode.
• The test covers browser mode, raw mode, and the unknown-engine case.

Contributor notes

Both dispatch switches live in cmd/search.go.