feat: add optional social_sentiment tool

[alexander-schneider]

Summary

• add an optional social_sentiment tool for structured cross-source social stock sentiment
• return normalized sentiment snapshots across Reddit, X/Twitter, and Polymarket
• register the tool only when ADANOS_API_KEY is set
• keep Dexter's existing financial_search flow unchanged
• add tests for normalization, partial source failures, registry gating, and the full paid lookback window

Why

Dexter already covers fundamentals, quotes, company news, and raw X search well.
What it does not currently have is a compact cross-source social stock sentiment snapshot for prompts like:

• "How much are Reddit users talking about TSLA right now?"
• "How hot is NVDA on X.com compared with AMD?"
• "How many active Polymarket bets are there on Microsoft right now?"
• "Are Reddit and X aligned on META?"

This PR fills that gap as a small optional tool without introducing provider selection or changing the current Financial Datasets-centered path.

Notes

• the tool is intentionally separate from financial_search
• it is env-gated and unavailable unless ADANOS_API_KEY is configured
• V1 focuses on Reddit, X/Twitter, and Polymarket only