fix: allow graph_store.llm to fully override LLM configuration

[danielguedesb]

Description

Allow graph_store.llm to fully override LLM configuration

Previously, graph_store.llm could only override the LLM provider but would still
use config.llm.config for configuration settings. This change allows graph_store.llm
to override both the provider and its configuration settings.

This enables more flexible LLM configuration when needed specifically for graph store operations.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[CLAassistant]

All committers have signed the CLA.