Add 'xai', 'x-ai', 'x.ai', 'grok' to _PROVIDER_PREFIXES so that
colon-prefixed model names (e.g. xai:grok-4.20) are stripped correctly
for context length lookups.
Cherry-picked from PR #9184 by @Julientalbot.
Adds xAI as a first-class provider: ProviderConfig in auth.py,
HermesOverlay in providers.py, 11 curated Grok models, URL mapping
in model_metadata.py, aliases (x-ai, x.ai), and env var tests.
Uses standard OpenAI-compatible chat completions.
Closes#7050
Julien Talbot