fix(web): add runtime availability check in web_search_tool handler

[liuhao1024]

What does this PR do?

Adds a runtime availability check at the beginning of web_search_tool() so that when the configured web search backend (e.g. ddgs) is unavailable at invocation time, the tool returns a clear, actionable error instead of silently falling through to provider-internal error handling.

Related Issue

Fixes #42011

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• tools/web_tools.py: Added _is_backend_available(backend) guard after _get_search_backend() in web_search_tool(). When the configured backend is not available, returns {"success": false, "error": "..."} with backend-specific installation instructions before the provider dispatch path is entered.
• tests/tools/test_web_tools_config.py: Added 3 new tests — unavailable ddgs backend, unavailable API-key backend, and empty-backend passthrough. Updated 2 existing tests to mock _is_backend_available.
• tests/tools/test_web_providers.py: Updated test_unconfigured_search_emits_top_level_error to match the new error message format.

How to Test

1. Set web.backend: ddgs in ~/.hermes/config.yaml
2. Ensure the ddgs package is NOT installed: pip uninstall ddgs
3. Call web_search tool — should return {"success": false, "error": "The configured web search backend 'ddgs' is not available. Install the package with: pip install ddgs"}
4. Run pytest tests/tools/test_web_tools_config.py tests/tools/test_web_providers.py tests/tools/test_web_providers_ddgs.py -v — all tests should pass

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Code Intelligence

• Analyzed: tools/web_tools.py::web_search_tool (callers: 1 via registry handler lambda)
• Blast radius: LOW — additive guard at handler entry; existing provider-internal error handling unchanged
• Related patterns: check_web_api_key() at get_definitions() time, _is_backend_available() shared with _get_capability_backend()

[alt-glitch]

Implements the fix for #42011 (web_search silently echoes tool call when backend package missing). Related work in availability-gate cluster: #31887, #11331.

[teknium1]

Thanks for the focused error-handling work. This is already implemented on current main through the provider-dispatch architecture.

• tools/web_tools.py:679-730 resolves the registered search provider and returns its structured result rather than using the inline dispatch path targeted by this PR.
• plugins/web/ddgs/provider.py:101-107 catches a missing ddgs import and returns {"success": false, "error": "ddgs package is not installed — run pip install ddgs"}`.
• tests/tools/test_web_providers_ddgs.py:134-150 covers that missing-package failure path.
• The registry dispatch arrived in b05253ceed5f9d139f4a7d8705f5c97fcf644a2c, included in v2026.5.16.

Automated hermes-sweeper review.