Resolve custom model limits automatically

[xiami762]

Summary

• leave custom model limit fields empty by default and omit unset values from the UI payload
• resolve custom model context/output limits from explicit values, existing config/catalog/runtime metadata, models.dev, then fallback
• add mocked and live models.dev coverage for common model ID inputs

Tests

• uv run pytest tests/server/routes/test_custom_provider_runtime.py
• FLOCKS_LIVE_TEST=1 uv run pytest tests/server/routes/test_custom_provider_runtime.py -k live_models_dev_auto_detects_current_inputs
• uv run ruff check flocks/server/routes/custom_provider.py tests/server/routes/test_custom_provider_runtime.py
• git diff --check

Note: webui build was not run because webui/node_modules is not installed in this worktree (tsc not found).