Refactor context handling with explicit naming

[iamzainhuda]

Summary: Previously used 0 index to identify default context and [1:] for sub modules, which makes it confusing to understand.

This diff refactors the context handling to use explicit naming:

• self._default_ctx: The default context for modules without specific configs
• self._submodule_ctxs: List of contexts for submodule-specific configurations
• self._ctxs: Combined list for iteration over all contexts

Differential Revision: D88995022

[meta-codesync[bot]]

@iamzainhuda has exported this pull request. If you are a Meta employee, you can view the originating Diff in D88995022.