#### What this PR does / why we need it

- Refined error messages for better clarity when credential resolution fails.
- Introduced `ErrNoIndirectCredentials` for clear distinction between direct and indirect resolution failures.
- Enhanced `ToGraph` to handle `nil` configurations and added fallback logic for indirect resolution.
- Updated method signatures to support the new `GraphResolver` interface.
- Adjusted indirect resolution logic to signal failure without excessive verbosity.
- Incorporated tests to validate updated error handling and resolution workflows.

#### Which issue(s) this PR fixes
Improves the readability and maintainability of credential graph resolution errors and ensures consistency across direct and indirect resolution paths.

Signed-off-by: Jakob Möller <jakob.moeller@sap.com>

#### What this commit does and why we need it

- Removed implicit initialization of `nil` configurations in `ToGraph` implementation.
- Updated tests to pass a valid configuration explicitly.
- This change enforces clearer and more explicit usage patterns by requiring valid configuration objects to be provided.

This enhances code clarity and prevents potential misuse of the `ToGraph` function by relying on explicit input parameters.

Signed-off-by: Jakob Möller <jakob.moeller@sap.com>

#### What this commit does and why we need it

- Refactored `resolveFromGraph` to streamline handling of indirect resolution fallback.
- Consolidated condition checks to avoid redundant error handling.
- Ensures fallback to repository plugin provider is only attempted when direct resolution fails with `ErrNoDirectCredentials`.

This simplifies the resolution logic, improves code readability, and reduces unnecessary branching during credential resolution.

Signed-off-by: Jakob Möller <jakob.moeller@sap.com>