fix(flow): handle terminal actions for unloaded flows

[vchaplygim]

Description of the Change

Problem

Flows can exist in the database but be absent from the in-memory flow map when worker loading fails at startup. One reproducible case is a flow whose saved provider name no longer exists after the provider was renamed or deleted.

Before this change, terminal actions such as finishFlow, stopFlow, REST PUT /flows/{id} with finish/stop, and GraphQL deleteFlow depended on the in-memory worker path. If the worker was not loaded, these actions returned flow not found or a generic internal error, leaving the flow stuck and its recorded container orphaned.

Solution

This PR adds a DB-backed fallback in the flow controller for unloaded flows:

• FinishFlow now verifies the DB row exists, removes recorded containers, marks the flow finished, publishes FlowUpdated, and returns success when the worker is missing.
• StopFlow now verifies the DB row exists, removes recorded running/starting containers, marks active unloaded flows waiting, publishes FlowUpdated, and returns success.
• REST stop/finish actions now call controller methods directly instead of requiring GetFlow first.
• GraphQL deleteFlow now routes through controller FinishFlow before deleting the DB row, closing the same cleanup gap.
• The fix does not attempt to repair provider mappings or resume flows whose provider is missing.

Closes #360

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Configuration change
• 🧪 Test update
• 🛡️ Security update

Areas Affected

• Core Services (Frontend UI/Backend API)
• AI Agents (Researcher/Developer/Executor)
• Security Tools Integration
• Memory System (Vector Store/Knowledge Base)
• Monitoring Stack (Grafana/OpenTelemetry)
• Analytics Platform (Langfuse)
• External Integrations (LLM/Search APIs)
• Documentation
• Infrastructure/DevOps

Testing and Verification

Test Configuration

PentAGI Version: local build from this branch, image local/pentagi:flow-fallback-fix
Docker Version: 29.4.3
Host OS: macOS Darwin 25.2.0
LLM Provider: custom OpenAI-compatible test gateway, model openrouter/z-ai/glm-5.2
Enabled Features: []