fix(providers): add iteration cap to performAgentChain to prevent infinite loops

[manusjs]

What

Adds a hard iteration cap to performAgentChain and escalating error behaviour in the repeating-detector to prevent infinite loops when a model repeatedly calls the same tool.

Fixes #175

Root Cause

performAgentChain in backend/pkg/providers/performer.go is an unbounded for {} loop. When the repeating detector fires it returns (message, nil) — a successful response — so wantToStop is never set and the loop continues. Production logs show 4800+ repeating-detector warnings in a single session.

Changes

• performer.go: Added maxAgentChainIterations = 100 cap. Loop returns an error if exceeded.
• Repeating detector: After N consecutive repeating detections (maxRepeatingBeforeAbort = 5) returns an actual error instead of a nil-error message, so the error path in performAgentChain fires.
• performer_test.go (new): TestAgentChainIterationCap — mocks a chain that never stops and asserts error is returned after maxAgentChainIterations.