Fix #387: Fix git clone auth by honoring configured git account username

[lukababu]

Closes #387

Why

Private repo clones can fail on providers that validate HTTP basic-auth usernames (for example GitLab deploy tokens, Bitbucket, and GitHub fine-grained PATs).
Komodo currently resolves a token but loses the configured account username before URL construction, so clone URLs fall back to token:<token>@.... That causes HTTP Basic: Access denied for providers that require a specific username.

What changed

• Updated Core git credential resolution to return credentials in username:token form (while preserving existing token values that already contain : for backward compatibility).
• Updated Periphery fallback credential resolution to do the same when Core does not provide a token.
• Added comments documenting the auth flow and compatibility behavior.

Behavior

Before:

• Clone URL often became https://token:<token>@<provider>/<repo>
  After:
• Clone URL uses configured account username:
  • https://<configured-username>:<token>@<provider>/<repo>

Backward compatibility

• Existing setups that already use the v1.19.2 workaround (token field manually set to username:token) continue to work unchanged.

Validation

• cargo check -p komodo_core passes
• cargo check -p komodo_periphery passes
• cargo clippy -p komodo_core -p komodo_periphery --no-deps passes
• Reproduced failing GitLab deploy-token clone case before patch
• NEEDS TO BE DONE - Verify clone succeeds after patch with same provider/account config