fix: align the credential functions to be the same

[bryanadenhq]

Description

Aligns list_credentials with check_agent so both functions follow the same credential resolution steps: loading shell env vars, resolving spec-name aliases, and surfacing flat-file credentials written by the GUI.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #6074

Changes Made

• Call ensure_credential_key_env() at the start of list_credentials to match check_agent behavior
• Resolve CREDENTIAL_SPECS aliases (e.g. gmail_oauth → google) before filtering accounts by credential_id
• Read flat-file credentials directly from EncryptedFileStorage to include GUI-saved credentials that LocalCredentialRegistry skips

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (cd core && pytest tests/)
• Lint passes (cd core && ruff check .)
• Manual testing performed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my 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