Add Parallel Web Search Integration to deepagents-cli

[NormallyGaussian]

Summary

Adds Parallel web search as an alternative to Tavily and exposes both as separate tools.

Design Decision: Separate Tools

We expose tavily_search() and parallel_search() as separate tools rather than a unified interface because:

Different Input Signatures:

• Tavily: query: str - single search query
• Parallel: queries: list[str], objective: str | None - multi-query with optional goal

Different Output Formats:

• Tavily: Includescontent field (string), relevance score and query
• Parallel: Includes excerpts field (array of markdown strings), publish_date, and search_id

Different Use Cases:

• Tavily: Single-query search with topic filtering ("general", "news", "finance")
• Parallel: Multi-query research optimized for LLM synthesis

Normalizing into one interface would lose functionality or create a confusing API.

Implementation

• Conditionally registered based on API keys
• Tavily still takes priority when both keys are set
• Separate approval configs for each tool

Testing

Added some tests; they all pass. I also tested manually with the Parallel API key enabled.

Breaking Changes

None. Existing web_search() maintained as backward-compatible wrapper which still uses Tavily.

[eyurtsev]

@NormallyGaussian we're holding off on additional integrations for now until we expose a proper way to configure the integrations. I'll keep the PR open and will ping here once ready