[GuideLLM Refactor] backend package updates, rewrites, and tests expansion (#355)

## **Summary**

Refactors the backend package to introduce a new architecture that
supports the scheduler refactor. The changes include new object models
for requests/responses, adjustments to the registry-based backend
system, and integration with the scheduler interface. The refactor
replaces the previous response/streaming model with a more flexible
request-response pattern that supports timing measurements and
distributed execution.

## **Details**

- **New object models**:
Introduced `GenerationRequest`, `GenerationResponse`,
and `GenerationRequestTimings` in `objects.py` to standardize data flow
between scheduler and backends
- **Backend architecture refactor**: Redesigned `Backend` base class to
extend `RegistryMixin` and `BackendInterface`, enabling automatic
registration and scheduler integration
- **Process lifecycle management**:
Added `process_startup()` and `process_shutdown()` methods to support
distributed worker processes with proper resource management
- **OpenAI backend modernization**: Rewrote `OpenAIHTTPBackend` with
improved error handling, streaming support, beginning support for
multimodal content processing, and configuration management
- **Registry system**: Implemented automatic backend registration using
decorators, replacing manual registry management
- **Scheduler integration**: Backends now
implement `BackendInterface` with `resolve()` method for processing
generation requests with timing metadata
- **Removed deprecated modules**: Eliminated `response.py` with its
streaming response types in favor of the new object model
- **Enhanced test coverage**: Added comprehensive unit tests for all new
components with smoke, sanity, and regression test categories

## **Test Plan**

- Execute new backend module test suites covering smoke, sanity, and
regression scenarios
- Verify OpenAI backend functionality with mocked HTTP responses for
both streaming and non-streaming modes
- Test backend registry and factory pattern functionality
- Validate integration with scheduler interfaces and timing measurement
- Ensure proper resource management during process startup/shutdown
cycles

## **Related Issues**

- Part of the larger scheduler refactor initiative

---

- [x] "I certify that all code in this PR is my own, except as noted
below."

## **Use of AI**

- [x]  Includes AI-assisted code completion
- [x]  Includes code generated by an AI application
- [] Includes AI-generated tests (NOTE: AI written tests should have a
docstring that includes `## WRITTEN BY AI ##`)

Author: markurtz