fix(codecompanion): support editor_context backend for MCP resource vars

[mahsank]

Description

This PR fixes CodeCompanion v19 compatibility for MCP resource variables in mcphub.nvim.

Root cause

mcphub.nvim assumed config.interactions.chat.variables always exists.
On CodeCompanion v19, this table can be missing, which causes a startup crash when make_vars = true (pairs() called on nil).

What this changes

• Adds runtime backend detection for resource variable registration:
  • config.interactions.chat.variables (legacy backend)
  • config.interactions.shared.editor_context (CodeCompanion v19+ backend)
  • config.interactions.chat.editor_context (fallback)
• Preserves existing user-facing identifiers and syntax:
  • #{mcp:...}
• Keeps backward compatibility with older CodeCompanion layouts.

Related Issue(s)

• Fixes Crash on startup with codecompanion v19 — nil passed to pairs() in variables.lua #275
• Fixes odecompanion extension: make_vars crashes — config.interactions.chat.variables is nil #278

Screenshots

N/A (no UI changes)

Checklist

• I've read the contributing guidelines and have adhered to them in this PR
• I've updated the README and/or relevant docs pages (N/A: no documentation source changes)
• I've run make test to ensure all tests pass (see Notes)
• I've run make format to format the code (see Notes)
• I've run make docs to update the vimdoc pages (N/A: no documentation source changes)

Notes

• make format fails locally in pre-existing/unrelated files (outside this PR), due to parser errors around continue labels.
• make test fails locally in an unrelated baseline test:
  • tests/native/neovim/files/edit_file/test_edit_ui.lua | reject_all_functionality
  • Can not use child.lua_func because child process is blocked
• The failure is outside files changed by this PR.
• The modified file in this PR is:
  • lua/mcphub/extensions/codecompanion/variables.lua