Register the bare model id 'claude-opus-5' to the Vertex AI route.

[copybara-service]

Register the bare model id 'claude-opus-5' to the Vertex AI route.

WHY
lf.LanguageModel.get('claude-opus-5') returns the direct Anthropic API
client instead of the Vertex AI one. vertexai.py binds Anthropic models by
iterating anthropic.SUPPORTED_MODELS and registering only those entries
whose provider is 'VertexAI'. The Claude Opus 5 entry declares
provider='Anthropic', so that loop skips it and the bare id stays bound to
the direct-API class registered by anthropic.py. Callers that select a model
purely by bare id therefore get the wrong transport. The neighbouring
claude-opus-4-6 / 4-7 / 4-8 ids avoid this because each is listed in an
explicit override block; Claude Opus 5 was absent from that block.

WHAT
One line added to _register_vertexai_models():

lf.LanguageModel.register('claude-opus-5', VertexAIClaude5Opus)

HOW
The line extends the existing override block that encodes the convention
"bare model ids resolve as VertexAI (primary use case)".
VertexAIClaude5Opus is already defined in this module with
model = 'claude-opus-5'; it was declared but never registered, so no new
class, import, dependency or BUILD change is required.

Ordering is load-bearing: _register_vertexai_models() applies these explicit
overrides after the SUPPORTED_MODELS loops, so the override replaces the
earlier direct-API binding rather than being shadowed by it.

Only the bare id is registered. The sibling opus-4-x entries additionally pin
an '@latest' alias to the direct API; no such alias is defined for Opus 5, so
no corresponding line is added here.