Migrate Manager to use common UnifiedConfig infrastructure

[HyeockJinKim]

Motivation

Refactor Manager's UnifiedConfig to use the consolidated common config classes and apply the new Annotated + BackendAIConfigMeta documentation pattern for better config documentation and versioning.

Required Features

1. Use common config imports (completed in BA-3768)

• EtcdConfig, OTELConfig, ServiceDiscoveryConfig, PyroscopeConfig imported from ai.backend.common.configs

2. Apply new documentation pattern to Manager-specific configs

Convert all config fields to use:

field_name: Annotated[
    FieldType,
    Field(default=...),
    BackendAIConfigMeta(
        description="...",
        added_version="X.Y.Z",
        example=ConfigExample(local="...", prod="..."),
    ),
]

Files to modify:

• src/ai/backend/manager/config/unified.py - All Manager config classes

Config classes to update:

• ManagerConfig
• SchedulerConfig
• APIConfig
• RedisConfig (Manager-specific)
• Other Manager-specific configs

Impact

• Manager component
• Config documentation generation
• Version tracking for config fields

Testing Scenarios

• Manager starts correctly
• All existing functionality preserved
• get_field_meta() returns correct metadata for all fields
• Config loading from TOML/etcd works correctly

JIRA Issue: BA-3769