Adding function to fetch website

[ribomo]

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[Copilot]

Pull request overview

This PR adds a new Exa-backed web_fetch tool (alongside the existing web_search) to allow fetching bounded Markdown content for a single approved HTTP/HTTPS URL, while refactoring Exa MCP client code into a shared provider module.

Changes:

• Introduces WebFetchTool (web_fetch) with URL validation + explicit approval gating, and registers it by default when network tools are enabled.
• Refactors Exa MCP client logic into a shared _ExaMcpClient, adding ExaFetchClient alongside ExaSearchClient.
• Updates tests, registry behavior assertions, and documentation to cover both web tools and the network opt-out behavior.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test_web_search.py	Adds coverage for ExaFetchClient and WebFetchTool, and updates imports for the new module layout.
tests/test_tools.py	Verifies web_fetch is included in tool definitions and toggled by enable_network.
tests/test_agent.py	Verifies agents expose/disable both web tools via the network toggle.
README.md	Documents “Web access” (search + fetch) and the shared opt-out env var.
plain_agent/tools/web/search.py	Updates search tool to use the new shared Exa provider module.
plain_agent/tools/web/providers/exa.py	Refactors Exa MCP request/response handling and adds ExaFetchClient.
plain_agent/tools/web/providers/init.py	Exposes provider clients/errors for web tools.
plain_agent/tools/web/fetch.py	Implements the new model-facing web_fetch tool, including URL validation and approval gating.
plain_agent/tools/web/init.py	Exports the approved web tools (WebSearchTool, WebFetchTool).
plain_agent/tools/web_search/init.py	Removes the old web_search package export surface after the module move.
plain_agent/tools/registry.py	Registers both web_search and web_fetch when network tools are enabled.
.env.example	Updates the network-tools comment to mention both search and fetch.

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