feat(llm): add compatible and vertex provider entrypoints

[nexxeln]

Summary

• add a generic Anthropic Messages-compatible package entrypoint and compose the named Anthropic provider from it
• add native Vertex Gemini and Vertex Anthropic entrypoints with project/location endpoint derivation, express API keys, OAuth tokens, and lazy ADC
• support current global/multi-region/regional hosts and tuned Gemini endpoints/... deployments
• document the package-like entrypoints and cover their executable request shapes

Testing

• bun test from packages/llm (328 passed, 29 skipped)
• bun test from packages/core (1287 passed)
• bun typecheck from packages/llm and packages/core
• repository typecheck via the push hook

Closes #34998
Closes #34999