feat: add support for out-of-tree terminal providers

[ThomasK33]

Add support for out-of-tree terminal providers

This PR allows creating custom terminal providers for claudecode.nvim by passing a table with required functions instead of using the built-in providers. This enables integration with any terminal plugin.

Key changes:

• Added validation and enhancement of custom provider tables
• Implemented graceful fallback to native provider if custom provider is invalid or unavailable
• Added auto-generation of optional functions for custom providers
• Updated documentation with detailed examples of creating custom providers
• Added comprehensive tests for custom provider functionality

The terminal provider configuration now accepts a table with required functions:

• setup: Initialize the terminal provider
• open: Open terminal with command and environment
• close: Close the terminal
• simple_toggle: Simple show/hide toggle
• focus_toggle: Smart toggle (focus if not focused, hide if focused)
• get_active_bufnr: Return terminal buffer number
• is_available: Check if the provider can be used

Optional functions are auto-generated if not provided:

• toggle: Defaults to calling simple_toggle for backward compatibility
• _get_terminal_for_test: For testing purposes

[ThomasK33]

• feat: add support for out-of-tree terminal providers #91 👈 (View in Graphite)
• main

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