Bugfix #353 : Improve adapter discovery.

[zyaga]

DISCLAIMER: Claude AI helped put together this PR.

Original Problem

When you opened a test file that belonged to a different project/directory than your current working directory (cwd), neotest would fail to find an adapter for that file. This happened because:

• On startup, neotest only registered adapters for projects in the current working directory
• The _get_adapter method would search through already-registered adapters
• If no adapter was found, it would just return nil - it never attempted to discover new adapters

Real-World Scenario

Imagine you have:

• /home/user/project-a/ (your cwd when neotest starts)
• /home/user/project-b/test_file.py (a file you open later)

When you opened project-b/test_file.py, neotest would fail to recognize it as a test file because it never registered an adapter for project-b. The adapter discovery only happened:

• At startup for your cwd
• When you changed directories (DirChanged event)

The Fix

We modified _get_adapter to implement lazy adapter discovery:

• First attempt: Search existing registered adapters (original behavior)
• If not found: Call _update_adapters(dir) for the file's parent directory
• Second attempt: Search again with newly registered adapters

-- First attempt to find adapter
local found_id, found_adapter = find_adapter()
if found_id then
  return found_id, found_adapter
end

-- If no adapter is found and client is started, update adapters and try again
if self._started then
  local dir = lib.files.is_dir(position_id) and position_id or lib.files.parent(position_id)
  self:_update_adapters(dir)  -- Discover and register adapters for this directory
  return find_adapter()        -- Try again
end

Benefits

✅ On-demand adapter discovery: Adapters are now discovered when needed, not just at startup
✅ Multi-project support: You can work with test files from multiple projects without restarting
✅ Reuses existing infrastructure: Leverages the existing _update_adapters method instead of duplicating logic
✅ Minimal performance impact: Only triggers when an adapter isn't found
✅ Safe: Only runs after client has started to avoid initialization issues

What This Enables

Users can now:

• Open test files from any project directory
• Work across multiple project roots simultaneously
• Have neotest automatically discover and register the appropriate adapter for each file
• Not worry about their current working directory when opening test files

Fixes #353