fix(web): discover provider plugins before web dispatch

[timothykersten]

Summary

• Ensure web provider plugins are discovered before search/extract/crawl registry dispatch.
• Prevent configured plugin backends such as brave-free and firecrawl from appearing unavailable when web tools are called outside normal CLI plugin initialization.
• Add regression coverage for direct web search dispatch.

Test plan

• python -m pytest tests/tools/test_web_tools_config.py::TestWebSearchSchema::test_web_search_discovers_provider_plugins_before_registry_lookup -q -o 'addopts='

Operational context

This is the second local carried hotfix from Tim/Petra's production Hermes instance. Kept separate from #27680 so the SMTPS fix PR remains narrowly scoped.

[alt-glitch]

Duplicate of #27700 and #27584 — all three PRs add _ensure_plugins_discovered() to web_tools.py dispatch sites. Fixes #27683/#27580.

[timothykersten]

Thanks — confirmed. This PR is a duplicate of #27700 and #27584 and fixes the same underlying issues (#27683/#27580). Closing this one rather than adding noise. I’ll track the existing upstream PRs/issues instead.