feat: Support project-level clangd path configuration via project.yml

[code4lala]

Summary

This PR adds support for configuring language server-specific settings (such as clangd path) at the project level through .serena/project.yml, previously this configuration was only possible globally via
~/.serena/serena_config.yml.

Background

Previously, users could only specify custom clangd paths using ls_specific_settings.cpp.ls_path in the global ~/.serena/serena_config.yml file. This limitation meant that different projects requiring different clangd
versions or paths could not be configured independently, forcing users to either:

• Use the same clangd for all projects, or
• Manually modify the global configuration when switching between projects

Changes

This PR introduces the following changes:

1. src/serena/config/serena_config.py:

   • Added ls_specific_settings field to the ProjectConfig dataclass
   • Updated _from_dict() method to properly load ls_specific_settings from YAML configuration

2. src/serena/project.py:

   • Modified LanguageServerFactory creation logic to merge ls_specific_settings from both global (SerenaConfig) and project (ProjectConfig) configurations
   • Project-level settings take precedence over global settings, allowing per-project overrides

Configuration Example

Users can now configure clangd path in their project's .serena/project.yml:

ls_specific_settings:
  cpp:
    ls_path: /path/to/project-specific/clangd

The configuration hierarchy is:

• Global ~/.serena/serena_config.yml (base configuration)
• Project .serena/project.yml (overrides global settings)

Testing

The implementation has been verified by:

1. Removing ls_specific_settings.cpp.ls_path from global ~/.serena/serena_config.yml
2. Adding the clangd path configuration to a project's .serena/project.yml
3. Running serena project index --log-level DEBUG and confirming:
   • The custom clangd path is correctly loaded from the project configuration
   • Clangd starts successfully using the specified path without attempting to download the default version

Impact

This change improves the developer experience by:

• Enabling project-specific language server configurations
• Allowing different projects to use different clangd versions or paths
• Reducing the need to modify global configuration when working with multiple projects
• Making project configurations more self-contained and portable

[code4lala]

@opcode81 This PR is ready for review! Thanks in advance for taking a look. 🙏 Please let me know if any updates are needed.