fix(benchmarks): scope target name uniqueness to cluster and add name filter

[JLCode-tech]

Description

Fixes benchmark target name collision when multiple clusters register endpoints with identical route names (e.g. via awsbnkctl discovering mcp-<namespace>-<route> on separate clusters).

Changes

1. Database Migration (v2_156):
   • Replaces the global unique constraint on benchmark_targets.name with composite unique constraint uq_benchmark_targets_cluster_name on (cluster_id, name).
   • Adds lookup index ix_benchmark_targets_name on name for fast filtering.
2. Backend ORM, Services & Schemas:
   • Updates BenchmarkTarget with composite UniqueConstraint and cluster_name property.
   • Updates BenchmarkTargetService duplicate validation to check (cluster_id, name).
   • Adds name exact-match query filter to list_targets.
   • Eager-loads cluster relationship via joinedload in list_targets and get_target to populate cluster_name without N+1 queries.
   • Exposes cluster_name: str | None on BenchmarkTargetResponse.
   • Updates GET /api/benchmarks/targets route to accept name: str | None = Query(None).
3. MCP Server Benchmark Tools:
   • Implements list_benchmark_targets and create_benchmark_target MCP tools in bnk_forge_mcp.tools.benchmarks.
   • Registers tools in MCP server and catalog with full governance audit tests.
4. Frontend UI (frontend-v2):
   • Updates BenchmarkTarget TypeScript type and listTargets API client.
   • In BenchmarkTargetsTab, adds a Cluster column in the targets table and displays the cluster badge in the detail header and Target details card.
   • In RunBenchmarkWizard and BenchmarkTrendsView, renders cluster name in target selection dropdowns so identically named targets across clusters are easily distinguishable.
5. Testing:
   • Alembic migration tests for SQLite and PostgreSQL: test_v2_156_benchmark_target_name_unique_per_cluster.py.
   • Component tests verifying cross-cluster duplicate creation, same-cluster rejection, rename conflict logic, and filtering: test_benchmark_target_uniqueness.py.
   • Route integration tests in test_routes_benchmarks.py.
   • MCP tool unit and catalog audit tests: test_benchmarks.py, test_tool_catalog.py, test_url_audit.py, test_server.py.
   • Frontend unit tests for hooks and UI components: useBenchmarks.test.ts and BenchmarkTargetsTab.test.tsx.