refactor(web): delete legacy tools/web_providers/ directory + migrate ABC tests

Removes the legacy in-tree provider scaffolding that PR #25182 fully
replaced with the plugin architecture:

  tools/web_providers/__init__.py        (6 lines)
  tools/web_providers/base.py            (89 lines — old ABCs)
  tools/web_providers/ARCHITECTURE.md    (73 lines — old design doc)

These were the staging-ground ABCs and provider modules that the
plugin migration absorbed. All seven web providers now implement the
single :class:`agent.web_search_provider.WebSearchProvider` ABC and
live under ``plugins/web/<vendor>/``. Nothing else in the tree imports
``tools.web_providers`` — verified via grep before deletion.

Test migration (tests/tools/test_web_providers.py)
--------------------------------------------------
Rewrote ``TestWebProviderABCs`` to test the new unified ABC at
:mod:`agent.web_search_provider`:

  - test_cannot_instantiate_abc_directly — abstract ``name`` + ``is_available``
  - test_concrete_search_only_provider_works — exercise default
    ``supports_extract=False`` / ``supports_crawl=False`` flags
  - test_concrete_multi_capability_provider_works — exercise all three
    capabilities, async extract supported (declared sync here for
    simplicity; real plugins like parallel + firecrawl use async)
  - test_search_only_provider_skips_extract_and_crawl — verify
    ``supports_*()`` flags default to False so search-only providers
    don't have to implement extract() or crawl()

The 9 other tests in the file (per-capability backend selection,
DEFAULT_CONFIG merge, dispatcher routing) test public helpers in
``tools.web_tools`` that still exist and pass unchanged.

agent/web_search_provider.py docstring updated to reflect that the
legacy ABCs no longer exist; the response-shape contract is preserved
bit-for-bit so external consumers see no behavioral change.

Net diff
--------
- tools/web_providers/ removed (-168 lines)
- tests/tools/test_web_providers.py rewritten ABC section (+78/-30 net,
  same coverage, new API)
- agent/web_search_provider.py docstring (-3/+5 lines)

Verified
--------
- 173/173 targeted web tests pass
- 12/12 ABC contract tests pass with the new interface
- No remaining grep hits for ``tools.web_providers`` outside of
  intentional historical references in plugin docstrings.

agent/web_search_provider.py

@@ -12,13 +12,14 @@ Providers live in ``<repo>/plugins/web/<name>/`` (built-in, auto-loaded as
 ``kind: backend``) or ``~/.hermes/plugins/web/<name>/`` (user, opt-in via
 ``plugins.enabled``).
 
-This ABC is the plugin-facing surface. The legacy
-:mod:`tools.web_providers.base` module retains its own ABCs for in-tree
-consumers that haven't migrated yet; over time those will all flow through
-this provider.
+This ABC is the SINGLE plugin-facing surface for web providers — every
+provider in the tree (brave-free, ddgs, searxng, exa, parallel, tavily,
+firecrawl) implements it. The legacy in-tree ``tools.web_providers.base``
+ABCs were deleted in PR #25182 along with the per-vendor inline helpers
+in ``tools/web_tools.py``; the response-shape contract documented below
+is preserved bit-for-bit so the tool wrapper does not have to translate.
 
-Response shape (mirrors the legacy contract in ``tools/web_providers/base.py``
-so the tool wrapper does not have to translate):
+Response shape (preserved from the legacy contract):
 
 Search results::