feat(messaging): Add inbox, conversation, search, and send message tools

[5queezer]

Summary

Add LinkedIn messaging support with 4 new MCP tools:

Tool	Description	readOnlyHint
get_inbox	List recent conversations from messaging inbox	true
get_conversation	Read a specific thread by username or thread_id	true
search_conversations	Search messages by keyword	true
send_message	Send a message to a LinkedIn user	false

Motivation

The server currently has 7 tools — all read-only. Messaging is a natural extension that enables AI assistants to read and respond to LinkedIn conversations. send_message is the first write operation in the server.

Implementation Notes

• Messaging tools don't follow the section-config pattern in fields.py — each tool method navigates directly
• All tools use _goto_with_auth_checks() and _extract_root_content(["main"]) for consistency
• send_message navigates to the user's profile, clicks the "Message" button, types into the contenteditable compose box, and clicks send
• send_message uses readOnlyHint: false to signal MCP clients to confirm before sending
• LinkedIn may restrict messaging to 1st-degree connections — the tool handles "Message" button absence gracefully

Files Changed

• linkedin_mcp_server/scraping/extractor.py — 4 new public methods
• linkedin_mcp_server/tools/messaging.py — new file with tool registration
• linkedin_mcp_server/server.py — import + register messaging tools
• linkedin_mcp_server/scraping/link_metadata.py — "messaging" reference kind
• tests/test_tools.py, tests/test_scraping.py, tests/test_link_metadata.py — test coverage
• README.md, manifest.json — documentation