feat(grpc-gcp): penalize retryable channel errors

[rahul2393]

With dynamic scaling enabled, a channel returning retryable errors (UNAVAILABLE / RESOURCE_EXHAUSTED) kept its low active-stream count and so kept attracting picks: the pool had no signal that a channel was unhealthy until its streams actually piled up, and scale-up never triggered for a pool that was failing rather than busy.

Change

• Error penalty on retryable failures. A call completing with UNAVAILABLE or RESOURCE_EXHAUSTED adds errorPenaltyStep (new pool option) of synthetic load to that channel for errorPenaltyDuration, capped at maxRpcPerChannel. The picker sees the penalty through getPickerLoad(), steering new picks toward healthy channels; expiry is lazy and lock-free.
• Scale-up integration. Penalty load counts toward the dynamic scale-up trigger, so a pool that is failing-but-idle can still grow.
• Single net-delta aggregate accounting. Every update to the shared penalty aggregate is computed under the channel lock and applied as one atomic delta, so lock-free readers never observe an intermediate state; a channel leaving the pool clears its contribution.
• Off by default (errorPenaltyStep = 0 disables); no allocations added on the per-RPC path.

[gemini-code-assist]

Code Review

This pull request introduces an error penalty mechanism for channels experiencing retryable errors (such as UNAVAILABLE or RESOURCE_EXHAUSTED). It adds configuration options for the penalty step and duration, tracks individual and aggregate penalty loads, and incorporates these penalties into the channel load calculations and scale-up signaling. Additionally, a comprehensive test suite has been added to verify the correctness of the error penalty logic under various scenarios. I have no feedback to provide.