feat: re-add web search with robust dual-source routing (#283)

[ASuresh0524]

Summary

Re-introduces web search (Serper API) and page fetching (Jina Reader) to the ReAct agent, replacing the reverted PR #232 with robust error handling and dual-source routing.

Key improvements over the original:

• Dynamic prompt adapts tool list based on whether SERPER_API_KEY is configured
• Graceful fallback when web_search is called without an API key (guides LLM to use leann_search)
• Error responses from Serper (401, rate limits, etc.) handled without crashing the agent
• visit_page errors (404, network failures) caught and surfaced as observations
• search_history entries now include a source field ("local" or "web") for debugging
• CLI gains --serper-api-key and --jina-api-key flags

Files changed:

• packages/leann-core/src/leann/web_search.py — re-added with typo fix
• packages/leann-core/src/leann/react_agent.py — dual-source routing, dynamic prompts, error handling
• packages/leann-core/src/leann/cli.py — new CLI args, source tags in search history display
• tests/test_react_dual_source.py — 13 tests covering all scenarios from issue ReAct agent: verify dual-source routing (local vs web) end-to-end #283

Test plan

• Prompt includes all 3 tools when Serper key is present
• Prompt shows only leann_search when no key
• Local-only routing works correctly
• Web-only routing works correctly (mocked)
• Mixed routing (local → web) works correctly
• Web results properly formatted as observations
• visit_page content truncated to 15k chars
• Local search results include scores
• No API key → graceful fallback message, no crash
• Invalid API key → error handled, agent continues
• 404 page → error string, agent continues
• Max iterations with mixed sources → still produces answer
• search_history entries include source field

All 13 tests pass locally (193s).

Closes #283

[ASuresh0524]

if passes, this is good