FEAT: Allow creation and display of RoundRobinTarget in GUI

[jsong468]

Description

Improves the GUI experience for RoundRobinTarget across three surfaces: the Target Configuration table, the Create Target dialog, and the Chat window target badge. Also fixes duplicate inner targets in TargetsInitializer caused by overlapping .env entries.

Target Configuration Table

• Expandable inner targets: RoundRobinTarget rows show a chevron (▶/▼) in the Type column that expands to reveal sub-rows for each inner target, showing their type, model/deployment name, endpoint, modalities, capabilities, and weight.
• Active target summary: The pinned active target row at the top also supports expand/collapse, sharing state with the main table row.
• Model column: Shows the shared model_name only when all inner targets have the same deployment name; otherwise shows "—" to avoid misleading display.
• Extracted InnerTargetRows component: Sub-row rendering is shared between the active summary and main table to eliminate duplication.

Create Target Dialog

• RoundRobinTarget creation: Added RoundRobinTarget to the target type dropdown. When selected, the endpoint/model/auth fields are replaced with a target picker that lets users select existing targets from the registry.
• Compatibility pre-filtering: After the first target is selected, the dropdown filters to only show compatible targets (same target_type, underlying_model_name, temperature, top_p). Backend performs definitive validation on submit.
• Weight configuration: Each selected target has a weight input (default 1) and a remove button. Submit is disabled until ≥2 targets are selected.
• Hidden irrelevant hints: The .pyrit_conf auto-populate hint is hidden for RoundRobinTarget since users are picking existing targets, not configuring new ones.

Chat Window Badge & Tooltip

• Informative badge: Shows RoundRobinTarget (gpt-4o ×3) using underlying_model_name (the shared model) rather than a potentially inconsistent deployment name.
• Inner Targets section: Tooltip includes an "Inner Targets" section listing each target's type, deployment name, endpoint, and weight.

Backend Changes

• TargetInstance model: Added optional inner_targets: list[TargetInstance] field for composite target support.
• Target mapper: Recursively maps inner targets into the DTO. Hoists model_name only when all inner targets agree; hoists underlying_model_name when uniform.
• Target service: create_target_async handles RoundRobinTarget by resolving target_registry_names from the registry and delegating all validation to the RoundRobinTarget constructor.
• Initializer dedup: _auto_group_targets now deduplicates inner targets by ComponentIdentifier.hash before creating round-robin groups. This prevents duplicate entries when .env files have overlapping configurations that resolve to identical targets (e.g., two env var blocks pointing to the same endpoint + model + key). Previously, these duplicates wasted rotation slots in the round-robin without adding any load-balancing benefit.

Drift Guard

• Added TestFrontendBackendCompatibilitySync — a backend test that asserts TARGET_EVAL_PARAMS matches the expected set. If someone adds a behavioral param, this test fails with a message directing them to update the frontend isCompatible() function.

Tests and Documentation

Backend Tests

• Mapper (test_mappers.py): 4 tests — inner target population, model name hoisting when uniform, model name omission when divergent, null for non-composite targets.
• Service (test_target_service.py): 3 tests — registry name resolution with mocked constructor, <2 names error, unknown name error.
• Initializer (test_targets_initializer.py): 1 test — deduplication of identical targets in auto-grouping verifies that a duplicate target (same endpoint + model + key registered under a different name) is excluded from the round-robin group.
• Drift guard (test_target_service.py): 1 test — TARGET_EVAL_PARAMS matches frontend isCompatible() fields.

Frontend Tests

• TargetTable (TargetTable.test.tsx): 2 tests — expand button renders only for RoundRobinTarget, click expands sub-rows with inner target details.
• TargetBadge (TargetBadge.test.tsx): 2 tests — ×N count in display name, underlying_model_name preferred over model_name.
• CreateTargetDialog (CreateTargetDialog.test.tsx): 2 tests — RoundRobinTarget shows target picker (hides endpoint fields), Create button disabled with <2 selections.

Manual Testing

• Verified end-to-end behavior manually: target creation, table display, expand/collapse, badge tooltip, compatibility filtering, and dedup behavior (previously 5 inner targets reduced to 3 after dedup fix).