refactor(BA-4143): Remove slot-based device partitioning methods

[hhoikoo]

resolves #8425 (BA-4143)

Overview

Simplify ResourceAllocator by removing mode-specific partitioning logic. All allocation modes (SHARED, AUTO_SPLIT, MANUAL) now behave like SHARED, where agents see all devices with no reserved slots between them. This establishes a clean baseline before implementing BEP-1041's device-centric partitioning design.

Problem Statement

• The current multi-agent resource allocation uses slot-based configuration with complex _calculate_device_slot_* methods that tightly couple slot calculation with allocation modes
• Before implementing the new device-based approach from BEP-1041, we need to establish a clean baseline by removing this complexity
• This allows us to incrementally rebuild the partitioning logic with the new device-centric design

Implementation

Removed methods from ResourceAllocator:

• _calculate_device_slot() - the mode dispatch method
• _calculate_device_slot_shared() - SHARED mode calculation
• _calculate_device_slot_auto_split() - AUTO_SPLIT mode calculation
• _calculate_device_slot_manual() - MANUAL mode calculation
• _ensure_slots_are_not_overallocated() - MANUAL mode validation

Simplified methods:

• _calculate_device_slots() - now always returns full available slots (SHARED behavior)
• _calculate_resource_scaling_factor() - now always returns 1.0
• _calculate_agent_partition() - removed unused parameters

Test changes:

• Skipped TestAutoSplitMode and TestManualMode classes with BA-4143/BEP-1041 explanations
• Skipped partitioning tests in TestMultiDeviceScenarios
• Added TestAllocationModesFallbackToShared class with 3 tests verifying all modes behave identically

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Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[Copilot]

Pull request overview

Refactors multi-agent ResourceAllocator to remove slot-based/mode-specific device partitioning so that SHARED, AUTO_SPLIT, and MANUAL all expose full device resources as a baseline ahead of BEP-1041.

Changes:

• Removes AUTO_SPLIT/MANUAL partitioning logic in ResourceAllocator, making all modes behave like SHARED.
• Simplifies agent partition/scaling-factor computation accordingly.
• Skips mode-specific tests and adds new tests asserting all modes fall back to SHARED behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/ai/backend/agent/resources.py	Removes mode-specific slot partitioning and makes partition/scaling-factor SHARED-only baseline.
tests/unit/agent/test_resource_allocation.py	Skips AUTO_SPLIT/MANUAL partitioning tests and adds fallback-to-SHARED tests.

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