fix: reject blank model names before registration

[loyce-cheng]

Description

When a model is configured through environment variables without MLSERVER_MODEL_NAME, it can be loaded successfully under an empty name and cannot be addressed through the model API. Reject empty and whitespace-only names before model initialisation, load hooks, or registry changes.

Name validation happens at registration so that file-based loading can still infer a missing or empty name from its directory, preserving the behaviour introduced in #190. ModelSettings.name remains optional until that resolution has taken place; valid names are not modified.

Changes Made

• Add InvalidModelName, an MLServerError with status 422 and configuration guidance.
• Validate the resolved name at the start of MultiModelRegistry.load().
• Cover invalid names, environment-only configuration, absence of loading side effects, and directory-name fallback; update both model-settings reference pages.

Related Issues

Fixes #2382.

Validation

• Reproduced seven failing regression cases on unchanged production code, including the reported successful empty-name load.
• 61 related settings, registry, repository-loading and repository-handler tests passed on Python 3.12.14 / Windows, using a local launcher for the upstream SIGQUIT import and non-uvloop event-loop-policy fixture issues. These launcher changes are not part of this PR.
• Black, Flake8, git diff --check, and mypy on the two changed production modules with --follow-imports=silent passed.
• Full core mypy reports the same nine diagnostics on this branch and an untouched upstream checkout: Windows signal constants and existing CORS/Starlette type mismatches.

Checklist

• Code follows the project's style guidelines
• Related tests listed above pass with the documented local launcher
• Documentation is updated
• Code is reviewed by at least one other team member
• Behaviour change is documented: unresolved blank names now fail at registration

Additional Notes

The real-server repository test is blocked locally by Windows add_signal_handler support. Linux process/signal behaviour, the full runtime suites and the Python 3.9–3.12 CI matrix have not been validated locally.