feat(model-gateway): separate worker base URL from API path prefix

[luyuzhe111]

Summary

• Separate worker URL into base URL (for health checks) and api_path (for proxying).
• Added new tests and a CI workflow for rllm-model-gateway.

Type of change

• Feature
• Fix
• Docs
• Refactor
• Example / Project
• Infra / CI

What changed

• Separate worker URL into base URL (for health checks) and api_path (for proxying), sohttp://host:4000/v1 becomes url=http://host:4000 + api_path=/v1
• Auto-split legacy URLs via a Pydantic model validator for backward compatibility — existing callers passing http://host:4000/v1 as url continue to work without changes
• Simplify _build_url by stripping a fixed gateway prefix instead of parsing worker URLs at request time
• Add CI workflow for rllm-model-gateway unit tests

Validation

• pre-commit run --all-files
• Targeted tests: pytest ...
• Manual validation performed
• Not run (reason below)

Validation details:

• All 170 unit tests passing.

Breaking changes / migration notes

• None

Docs / examples

• Not needed
• Updated docs
• Updated examples
• Follow-up docs needed

Related issues / PRs

• Fixes #
• Related to #
• Stacked on / depends on #

Screenshots / logs