refactor: restructure resolver logic and improve type safety (#1323)

# What does this PR do?

- Modularized `resolve_impls` by extracting helper functions for
validation, sorting, and instantiation.
- Improved readability by introducing `validate_and_prepare_providers`,
`sort_providers_by_dependency`, and `instantiate_providers`.
- Enhanced type safety with explicit type hints (`Tuple`, `Dict`, `Set`,
etc.).
- Fixed potential issues with provider module imports and added error
handling.
- Updated `pyproject.toml` to enforce type checking on `resolver.py`
using `mypy`.

Signed-off-by: Sébastien Han <seb@redhat.com>

- [//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan

Run the server.

[//]: # (## Documentation)

Signed-off-by: Sébastien Han <seb@redhat.com>

pyproject.toml

@@ -75,11 +75,7 @@ docs = [
     "sphinxcontrib.mermaid",
     "tomli",
 ]
-codegen = [
-    "rich",
-    "pydantic",
-    "jinja2",
-]
+codegen = ["rich", "pydantic", "jinja2"]
 
 [project.urls]
 Homepage = "https://github.com/meta-llama/llama-stack"
@@ -163,3 +159,7 @@ exclude = [
 # packages that lack typing annotations, do not have stubs, or are unavailable.
 module = ["yaml", "fire"]
 ignore_missing_imports = true
+
+[[tool.mypy.overrides]]
+module = "llama_stack.distribution.resolver"
+follow_imports = "normal"                    # This will force type checking on this module