fix(router:PLA-638): stabilize network timeout classification

[0x666c6f]

Summary

• make network-level timeout classification deterministic under scheduler jitter
• keep upstream request context timeout slightly beyond failsafe timeout using a named slack constant
• add a repeated regression test to ensure timeout errors surface as failsafe timeout (not upstream timeout variants)

Changes

• add networkFailsafeTimeoutSlack constant and use it in Network.Forward timeout context wiring
• add TestNetwork_Forward_TimeoutClassificationDeterministic coverage with repeated assertions
• run full gate (make agent-gate) and focused timeout tests

Linear

• TICKET: https://linear.app/morpho-labs/issue/PLA-638/improve-rpc-reliability-for-router